Add LanguageTool grammar/spell checking with a settings dialog

Check the current file against a LanguageTool server (typically a local
instance) from a background thread. Issues are underlined in the editor
(red spelling, blue grammar) and listed in a bottom panel with one-click
fixes that re-align the remaining matches.

A menu bar (File / View / Settings) exposes a LanguageTool settings
dialog with discrete scheme / host / port / token / language fields, an
endpoint preview, and a Test connection button. Config stores the parts
and assembles the base URL. The token is sent as a bearer header.

Uses ureq with a statically-linked rustls backend, so both http and
https endpoints work with no new runtime library dependencies.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.lock

@@ -452,6 +452,12 @@ version = "1.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f2032f911046de80f0a198e0901378627c33f59ea0ac00e363d481118bd70a53"
 
+[[package]]
+name = "base64"
+version = "0.22.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72b3254f16251a8381aa12e40e3c4d2f0199f8c6508fbecb9d91f575e0fbb8c6"
+
 [[package]]
 name = "bit-set"
 version = "0.6.0"
@@ -1852,6 +1858,7 @@ dependencies = [
  "rfd",
  "serde",
  "serde_json",
+ "ureq",
  "zip",
 ]
 
@@ -2691,6 +2698,20 @@ dependencies = [
  "windows-sys 0.59.0",
 ]
 
+[[package]]
+name = "ring"
+version = "0.17.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a4689e6c2294d81e88dc6261c768b63bc4fcdb852be6d1352498b114f61383b7"
+dependencies = [
+ "cc",
+ "cfg-if",
+ "getrandom 0.2.17",
+ "libc",
+ "untrusted",
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "rustc-hash"
 version = "1.1.0"
@@ -2738,6 +2759,41 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "rustls"
+version = "0.23.42"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3c54fcab019b409d04215d3a17cb438fd7fbf192ee61461f20f4fe18704bc138"
+dependencies = [
+ "log",
+ "once_cell",
+ "ring",
+ "rustls-pki-types",
+ "rustls-webpki",
+ "subtle",
+ "zeroize",
+]
+
+[[package]]
+name = "rustls-pki-types"
+version = "1.15.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2f4925028c7eb5d1fcdaf196971378ed9d2c1c4efc7dc5d011256f76c99c0a96"
+dependencies = [
+ "zeroize",
+]
+
+[[package]]
+name = "rustls-webpki"
+version = "0.103.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "61c429a8649f110dddef65e2a5ad240f747e85f7758a6bccc7e5777bd33f756e"
+dependencies = [
+ "ring",
+ "rustls-pki-types",
+ "untrusted",
+]
+
 [[package]]
 name = "rustversion"
 version = "1.0.23"
@@ -3007,6 +3063,12 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6637bab7722d379c8b41ba849228d680cc12d0a45ba1fa2b48f2a30577a06731"
 
+[[package]]
+name = "subtle"
+version = "2.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "13c2bddecc57b384dee18652358fb23172facb8a2c51ccc10d74c157bdea3292"
+
 [[package]]
 name = "syn"
 version = "1.0.109"
@@ -3272,6 +3334,27 @@ version = "0.2.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ebc1c04c71510c7f702b52b7c350734c9ff1295c464a03335b00bb84fc54f853"
 
+[[package]]
+name = "untrusted"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8ecb6da28b8a351d773b68d5825ac39017e680750f980f3a1a85cd8dd28a47c1"
+
+[[package]]
+name = "ureq"
+version = "2.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "02d1a66277ed75f640d608235660df48c8e3c19f3b4edb6a263315626cc3c01d"
+dependencies = [
+ "base64",
+ "log",
+ "once_cell",
+ "rustls",
+ "rustls-pki-types",
+ "url",
+ "webpki-roots 0.26.11",
+]
+
 [[package]]
 name = "url"
 version = "2.5.8"
@@ -3565,6 +3648,24 @@ dependencies = [
  "web-sys",
 ]
 
+[[package]]
+name = "webpki-roots"
+version = "0.26.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "521bc38abb08001b01866da9f51eb7c5d647a19260e00054a8c7fd5f9e57f7a9"
+dependencies = [
+ "webpki-roots 1.0.9",
+]
+
+[[package]]
+name = "webpki-roots"
+version = "1.0.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7dcd9d09a39985f5344844e66b0c530a33843579125f23e21e9f0f220850f22a"
+dependencies = [
+ "rustls-pki-types",
+]
+
 [[package]]
 name = "wgpu"
 version = "22.1.0"
@@ -4393,6 +4494,12 @@ dependencies = [
  "synstructure",
 ]
 
+[[package]]
+name = "zeroize"
+version = "1.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e13c156562582aa81c60cb29407084cdb54c4164760106ab78e6c5b0858cf64e"
+
 [[package]]
 name = "zerotrie"
 version = "0.2.4"

Cargo.toml

@@ -14,6 +14,10 @@ zip = { version = "2", default-features = false, features = ["deflate"] }
 dirs = "5"
 # Native open/save dialogs via the XDG Desktop Portal (no GTK build dependency).
 rfd = { version = "0.15", default-features = false, features = ["xdg-portal", "async-std"] }
+# Blocking HTTP client for talking to a LanguageTool server. The rustls TLS
+# backend links statically (no new runtime .so deps), so both a plain-HTTP local
+# server and an HTTPS domain work.
+ureq = { version = "2", default-features = false, features = ["tls"] }
 
 [profile.release]
 opt-level = 2

README.md

@@ -84,6 +84,48 @@ The status bar shows a live **word count** for the current file, plus the net
 words added (or removed) to it since the current session started. The count
 updates as you type, including unsaved edits.
 
+## Grammar & spelling (LanguageTool)
+
+The editor can check the current file against a
+[LanguageTool](https://languagetool.org/) server — typically a **local
+instance**, so your manuscript never leaves your machine.
+
+1. Run a LanguageTool server. The standalone server listens on
+   `http://localhost:8010` by default (the official Docker image uses `:8081` —
+   just point the app at whichever you run):
+   ```sh
+   # Standalone (needs Java):
+   java -cp 'languagetool-server.jar' org.languagetool.server.HTTPServer --port 8010
+   # or via Docker:
+   docker run -d -p 8010:8010 --entrypoint java \
+     erikvl87/languagetool -cp languagetool-server.jar \
+     org.languagetool.server.HTTPServer --port 8010 --allow-origin '*'
+   ```
+2. Open **Settings ▸ LanguageTool…** (from the menu bar, or the **⚙** button on
+   the Grammar row) and set the connection:
+   * **Scheme** — `http` for a local server, `https` for a remote one (TLS is
+     supported).
+   * **Host / domain** — an IP address or hostname (default `localhost`).
+   * **Port** — the server's port (default `8010`).
+   * **Token** — optional; sent as an `Authorization: Bearer` header for a server
+     behind an auth proxy, or a premium API key. Leave blank for an
+     unauthenticated local server.
+   * **Language** — `auto` to detect, or a code like `en-US`, `en-GB`, `de-DE`.
+
+   Press **Test connection** to confirm the settings reach a working server. All
+   fields are remembered in the config.
+3. Open a file and press **✓ Check**. The check runs in the background (the UI
+   stays responsive) and issues appear two ways:
+   * **Underlines in the editor** — red for spelling, blue for grammar/style.
+   * **A "Grammar & spelling" panel** at the bottom listing each issue with its
+     explanation and suggested fixes. Click a suggestion to apply it; the
+     remaining underlines re-align automatically.
+
+Editing the text after a check hides the underlines (their positions would no
+longer be accurate) and disables the fix buttons until you re-check. The checker
+sends the raw file text, so occasional markdown tokens (e.g. a `#` heading
+marker) may be flagged.
+
 ## Header stripping on export
 
 Manuscript files often carry editorial notes and metadata above the prose that

src/app.rs

@@ -6,6 +6,12 @@ use eframe::egui;
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 
+/// Result of a background grammar check: the matches, or an error message.
+type LtCheckResult = Result<Vec<crate::langtool::Match>, String>;
+/// Channel payload from a background check: the text that was checked, paired
+/// with its result (so the app can confirm the buffer hasn't changed since).
+type LtCheckMsg = (String, LtCheckResult);
+
 pub struct App {
     config: Config,
     /// Ordered markdown file names (relative to the workspace).
@@ -33,6 +39,23 @@ pub struct App {
     /// Each file's word count captured when the workspace was opened, used as the
     /// per-file baseline for the "this session" delta.
     session_start_counts: HashMap<String, usize>,
+    /// Grammar/spelling issues from the last LanguageTool check.
+    lt_matches: Vec<crate::langtool::Match>,
+    /// The exact buffer text the current `lt_matches` were computed against;
+    /// underlines and replacements only apply while the buffer still equals it.
+    lt_checked_text: String,
+    /// One-line status for the grammar checker (issue count, error, or progress).
+    lt_status: String,
+    /// Receiver for an in-flight background check, if any.
+    lt_rx: Option<std::sync::mpsc::Receiver<LtCheckMsg>>,
+    /// Whether the grammar results panel is visible.
+    show_lt_panel: bool,
+    /// Whether the LanguageTool settings window is open.
+    show_settings: bool,
+    /// Receiver for an in-flight "Test connection" from the settings window.
+    settings_test_rx: Option<std::sync::mpsc::Receiver<Result<usize, String>>>,
+    /// Result line for the settings window's "Test connection" button.
+    settings_test_status: String,
 }
 
 impl App {
@@ -56,6 +79,14 @@ impl App {
             git_log: String::new(),
             is_repo: false,
             session_start_counts: HashMap::new(),
+            lt_matches: Vec::new(),
+            lt_checked_text: String::new(),
+            lt_status: String::new(),
+            lt_rx: None,
+            show_lt_panel: false,
+            show_settings: false,
+            settings_test_rx: None,
+            settings_test_status: String::new(),
         };
         app.open_workspace();
         app
@@ -82,6 +113,7 @@ impl App {
         self.buffer.clear();
         self.dirty = false;
         self.pending_delete = false;
+        self.clear_lt();
         self.session_start_counts = self.snapshot_counts();
         if !self.files.is_empty() {
             self.select(0);
@@ -147,11 +179,19 @@ impl App {
         }
     }
 
+    /// Discard any grammar-check results (they belong to a specific buffer).
+    fn clear_lt(&mut self) {
+        self.lt_matches.clear();
+        self.lt_checked_text.clear();
+        self.lt_status.clear();
+    }
+
     fn select(&mut self, idx: usize) {
         if self.selected == Some(idx) {
             return;
         }
         self.save_current();
+        self.clear_lt();
         if let Some(name) = self.files.get(idx) {
             let path = self.path_for(name);
             self.buffer = std::fs::read_to_string(&path).unwrap_or_default();
@@ -395,6 +435,135 @@ impl App {
         }
     }
 
+    /// Kick off a LanguageTool check of the current buffer on a background
+    /// thread. Results are delivered through `lt_rx` and picked up in `update`.
+    fn start_lt_check(&mut self, ctx: &egui::Context) {
+        if self.selected.is_none() {
+            self.lt_status = "Open a file to check".to_string();
+            return;
+        }
+        if self.lt_rx.is_some() {
+            return; // a check is already running
+        }
+        if self.config.languagetool_host.trim().is_empty() {
+            self.lt_status = "Set a LanguageTool host in Settings first".to_string();
+            self.show_lt_panel = true;
+            self.show_settings = true;
+            return;
+        }
+        let url = self.config.languagetool_base_url();
+        let token = self.config.languagetool_token.clone();
+        let mut language = self.config.languagetool_language.trim().to_string();
+        if language.is_empty() {
+            language = "auto".to_string();
+        }
+        let text = self.buffer.clone();
+        let (tx, rx) = std::sync::mpsc::channel();
+        self.lt_rx = Some(rx);
+        self.lt_status = "Checking…".to_string();
+        self.show_lt_panel = true;
+        let ctx = ctx.clone();
+        std::thread::spawn(move || {
+            let result = crate::langtool::check(&url, &language, &token, &text);
+            let _ = tx.send((text, result));
+            ctx.request_repaint();
+        });
+    }
+
+    /// Ping the configured server with a sample sentence to confirm the settings
+    /// work, without disturbing the editor's current results. Used by the
+    /// Settings dialog's "Test connection" button.
+    fn test_lt_connection(&mut self, ctx: &egui::Context) {
+        if self.settings_test_rx.is_some() {
+            return;
+        }
+        let url = self.config.languagetool_base_url();
+        let token = self.config.languagetool_token.clone();
+        let language = {
+            let l = self.config.languagetool_language.trim();
+            if l.is_empty() { "auto".to_string() } else { l.to_string() }
+        };
+        let (tx, rx) = std::sync::mpsc::channel();
+        self.settings_test_rx = Some(rx);
+        self.settings_test_status = "Testing…".to_string();
+        let ctx = ctx.clone();
+        std::thread::spawn(move || {
+            // A sentence with a deliberate error, so a working server returns ≥1 match.
+            let result =
+                crate::langtool::check(&url, &language, &token, "The team are ready to began.");
+            let _ = tx.send(result.map(|m| m.len()));
+            ctx.request_repaint();
+        });
+    }
+
+    /// Pick up a finished "Test connection" result for the Settings dialog.
+    fn poll_settings_test(&mut self) {
+        let received = self
+            .settings_test_rx
+            .as_ref()
+            .and_then(|rx| rx.try_recv().ok());
+        if let Some(result) = received {
+            self.settings_test_rx = None;
+            self.settings_test_status = match result {
+                Ok(_) => format!("✔ Connected to {}", self.config.languagetool_base_url()),
+                Err(e) => format!("✖ {e}"),
+            };
+        }
+    }
+
+    /// Poll for a finished background check and store its results.
+    fn poll_lt(&mut self) {
+        let received = self.lt_rx.as_ref().and_then(|rx| rx.try_recv().ok());
+        if let Some((text, result)) = received {
+            self.lt_rx = None;
+            match result {
+                Ok(matches) => {
+                    self.lt_status = match matches.len() {
+                        0 => "No issues found".to_string(),
+                        1 => "1 issue".to_string(),
+                        n => format!("{n} issues"),
+                    };
+                    self.lt_matches = matches;
+                    self.lt_checked_text = text;
+                }
+                Err(e) => {
+                    self.lt_matches.clear();
+                    self.lt_checked_text.clear();
+                    self.lt_status = e;
+                }
+            }
+        }
+    }
+
+    /// Apply replacement `rep_idx` of match `match_idx` to the buffer, then shift
+    /// the remaining matches so their highlights stay valid.
+    fn apply_replacement(&mut self, match_idx: usize, rep_idx: usize) {
+        // Only safe while the buffer still matches what was checked.
+        if self.buffer != self.lt_checked_text {
+            return;
+        }
+        let Some(m) = self.lt_matches.get(match_idx).cloned() else {
+            return;
+        };
+        let Some(replacement) = m.replacements.get(rep_idx).cloned() else {
+            return;
+        };
+        if m.end > self.buffer.len() || !self.buffer.is_char_boundary(m.start) {
+            return;
+        }
+        self.buffer.replace_range(m.start..m.end, &replacement);
+        self.dirty = true;
+        self.lt_checked_text = self.buffer.clone();
+
+        // The applied match overlaps its own region, so it is dropped here too.
+        remap_matches(&mut self.lt_matches, m.start, m.end, replacement.len());
+        self.lt_status = match self.lt_matches.len() {
+            0 => "No issues remaining".to_string(),
+            1 => "1 issue".to_string(),
+            n => format!("{n} issues"),
+        };
+    }
+
     fn reorder(&mut self, from: usize, mut to: usize) {
         if from >= self.files.len() || from == to {
             return;
@@ -495,6 +664,39 @@ impl App {
                     self.config.save();
                 }
             });
+            ui.add_space(2.0);
+            ui.horizontal(|ui| {
+                ui.label("Grammar:").on_hover_text(
+                    "Grammar & spelling via a LanguageTool server. \
+                     Configure it in Settings ▸ LanguageTool.",
+                );
+                let checking = self.lt_rx.is_some();
+                if ui
+                    .add_enabled(
+                        !checking && self.selected.is_some(),
+                        egui::Button::new("✓ Check"),
+                    )
+                    .on_hover_text("Check the current file's grammar and spelling")
+                    .clicked()
+                {
+                    self.start_lt_check(ctx);
+                }
+                if !self.lt_matches.is_empty()
+                    && ui.button("Clear").on_hover_text("Clear check results").clicked()
+                {
+                    self.clear_lt();
+                }
+                if ui
+                    .button("⚙")
+                    .on_hover_text("LanguageTool settings")
+                    .clicked()
+                {
+                    self.show_settings = true;
+                }
+                if !self.lt_status.is_empty() {
+                    ui.label(egui::RichText::new(&self.lt_status).weak());
+                }
+            });
             ui.add_space(4.0);
         });
 
@@ -653,6 +855,243 @@ impl App {
             });
     }
 
+    /// The application menu bar (File / View / Settings).
+    fn menu_bar(&mut self, ctx: &egui::Context) {
+        egui::TopBottomPanel::top("menubar").show(ctx, |ui| {
+            egui::menu::bar(ui, |ui| {
+                ui.menu_button("File", |ui| {
+                    if ui.button("📂 Open workspace…").clicked() {
+                        ui.close_menu();
+                        self.browse_workspace();
+                    }
+                    if ui.button("Export ODT").clicked() {
+                        ui.close_menu();
+                        self.export_odt();
+                    }
+                    ui.separator();
+                    if ui.button("Quit").clicked() {
+                        ui.close_menu();
+                        ctx.send_viewport_cmd(egui::ViewportCommand::Close);
+                    }
+                });
+                ui.menu_button("View", |ui| {
+                    if ui
+                        .checkbox(&mut self.config.show_preview, "Preview pane")
+                        .clicked()
+                    {
+                        self.config.save();
+                    }
+                    ui.checkbox(&mut self.show_lt_panel, "Grammar panel");
+                    ui.checkbox(&mut self.show_log, "Git log");
+                });
+                ui.menu_button("Settings", |ui| {
+                    if ui.button("LanguageTool…").clicked() {
+                        ui.close_menu();
+                        self.show_settings = true;
+                    }
+                });
+            });
+        });
+    }
+
+    /// Floating window for editing the LanguageTool connection settings.
+    fn settings_window(&mut self, ctx: &egui::Context) {
+        let mut open = self.show_settings;
+        let mut close_clicked = false;
+        egui::Window::new("LanguageTool settings")
+            .open(&mut open)
+            .resizable(false)
+            .collapsible(false)
+            .show(ctx, |ui| {
+                let mut save_now = false;
+                egui::Grid::new("lt_settings_grid")
+                    .num_columns(2)
+                    .spacing([10.0, 8.0])
+                    .show(ui, |ui| {
+                        ui.label("Scheme:");
+                        egui::ComboBox::from_id_salt("lt_scheme")
+                            .selected_text(self.config.languagetool_scheme.clone())
+                            .show_ui(ui, |ui| {
+                                for s in ["http", "https"] {
+                                    if ui
+                                        .selectable_value(
+                                            &mut self.config.languagetool_scheme,
+                                            s.to_string(),
+                                            s,
+                                        )
+                                        .clicked()
+                                    {
+                                        save_now = true;
+                                    }
+                                }
+                            });
+                        ui.end_row();
+
+                        ui.label("Host / domain:");
+                        let r = ui.add(
+                            egui::TextEdit::singleline(&mut self.config.languagetool_host)
+                                .hint_text("localhost or lt.example.com")
+                                .desired_width(230.0),
+                        );
+                        save_now |= r.lost_focus();
+                        ui.end_row();
+
+                        ui.label("Port:");
+                        let r = ui.add(
+                            egui::DragValue::new(&mut self.config.languagetool_port)
+                                .speed(1.0)
+                                .range(1..=65535),
+                        );
+                        save_now |= r.drag_stopped() || r.lost_focus();
+                        ui.end_row();
+
+                        ui.label("Token:");
+                        let r = ui.add(
+                            egui::TextEdit::singleline(&mut self.config.languagetool_token)
+                                .password(true)
+                                .hint_text("optional — auth proxy / API key")
+                                .desired_width(230.0),
+                        );
+                        save_now |= r.lost_focus();
+                        ui.end_row();
+
+                        ui.label("Language:");
+                        let r = ui.add(
+                            egui::TextEdit::singleline(&mut self.config.languagetool_language)
+                                .hint_text("auto, en-US, de-DE…")
+                                .desired_width(120.0),
+                        );
+                        save_now |= r.lost_focus();
+                        ui.end_row();
+                    });
+
+                ui.add_space(4.0);
+                ui.label(
+                    egui::RichText::new(format!(
+                        "Endpoint: {}/v2/check",
+                        self.config.languagetool_base_url()
+                    ))
+                    .weak()
+                    .monospace(),
+                );
+                ui.separator();
+
+                ui.horizontal(|ui| {
+                    let testing = self.settings_test_rx.is_some();
+                    if ui
+                        .add_enabled(!testing, egui::Button::new("Test connection"))
+                        .clicked()
+                    {
+                        self.config.save();
+                        self.test_lt_connection(ctx);
+                    }
+                    if ui.button("Close").clicked() {
+                        close_clicked = true;
+                    }
+                });
+                if !self.settings_test_status.is_empty() {
+                    ui.label(&self.settings_test_status);
+                }
+
+                if save_now {
+                    self.config.save();
+                }
+            });
+
+        let now_open = open && !close_clicked;
+        if self.show_settings && !now_open {
+            // Window is closing — persist and reset its transient state.
+            self.config.save();
+            self.settings_test_status.clear();
+        }
+        self.show_settings = now_open;
+    }
+
+    /// Bottom panel listing grammar/spelling issues with one-click fixes.
+    fn lt_panel(&mut self, ctx: &egui::Context) {
+        egui::TopBottomPanel::bottom("ltpanel")
+            .resizable(true)
+            .default_height(190.0)
+            .show(ctx, |ui| {
+                let stale = self.buffer != self.lt_checked_text;
+                ui.horizontal(|ui| {
+                    ui.label(egui::RichText::new("Grammar & spelling").strong());
+                    if !self.lt_status.is_empty() {
+                        ui.label(egui::RichText::new(&self.lt_status).weak());
+                    }
+                    if stale && !self.lt_matches.is_empty() {
+                        ui.label(
+                            egui::RichText::new("· edited since check — re-check to apply fixes")
+                                .weak()
+                                .italics(),
+                        );
+                    }
+                    ui.with_layout(egui::Layout::right_to_left(egui::Align::Center), |ui| {
+                        if ui.button("Hide").clicked() {
+                            self.show_lt_panel = false;
+                        }
+                    });
+                });
+                ui.separator();
+
+                if self.lt_matches.is_empty() {
+                    ui.label(
+                        egui::RichText::new(if self.lt_rx.is_some() {
+                            "Checking…"
+                        } else {
+                            "No issues to show."
+                        })
+                        .weak(),
+                    );
+                    return;
+                }
+
+                let mut apply: Option<(usize, usize)> = None;
+                egui::ScrollArea::vertical()
+                    .auto_shrink([false, false])
+                    .show(ui, |ui| {
+                        for (i, m) in self.lt_matches.iter().enumerate() {
+                            ui.horizontal_wrapped(|ui| {
+                                let (dot, col) = if m.spelling {
+                                    ("●", egui::Color32::from_rgb(0xE0, 0x40, 0x40))
+                                } else {
+                                    ("●", egui::Color32::from_rgb(0x3B, 0x82, 0xF6))
+                                };
+                                ui.label(egui::RichText::new(dot).color(col));
+                                if let Some(snippet) = self.lt_checked_text.get(m.start..m.end) {
+                                    ui.label(
+                                        egui::RichText::new(format!("“{snippet}”")).strong(),
+                                    );
+                                }
+                                ui.label(&m.message);
+                            });
+                            ui.horizontal_wrapped(|ui| {
+                                ui.add_space(16.0);
+                                if m.replacements.is_empty() {
+                                    ui.label(
+                                        egui::RichText::new("(no suggestion)").weak().italics(),
+                                    );
+                                } else {
+                                    for (j, rep) in m.replacements.iter().take(8).enumerate() {
+                                        if ui
+                                            .add_enabled(!stale, egui::Button::new(rep).small())
+                                            .clicked()
+                                        {
+                                            apply = Some((i, j));
+                                        }
+                                    }
+                                }
+                            });
+                            ui.separator();
+                        }
+                    });
+
+                if let Some((i, j)) = apply {
+                    self.apply_replacement(i, j);
+                }
+            });
+    }
+
     fn central(&mut self, ctx: &egui::Context) {
         egui::CentralPanel::default().show(ctx, |ui| {
             match self.selected {
@@ -748,16 +1187,31 @@ impl App {
             .unwrap_or(12.0);
         let size = (base * (1.0 + self.config.editor_zoom / 100.0)).max(4.0);
 
+        // Underline grammar/spelling matches, but only while the buffer still
+        // equals the text they were computed against (edits invalidate offsets).
+        let ranges: Vec<(usize, usize, bool)> = if self.buffer == self.lt_checked_text {
+            self.lt_matches
+                .iter()
+                .map(|m| (m.start, m.end, m.spelling))
+                .collect()
+        } else {
+            Vec::new()
+        };
+
         egui::ScrollArea::vertical()
             .id_salt("editor")
             .auto_shrink([false, false])
             .show(ui, |ui| {
+                let mut layouter = move |ui: &egui::Ui, text: &str, wrap_width: f32| {
+                    let job = build_editor_job(ui, text, size, &ranges, wrap_width);
+                    ui.fonts(|f| f.layout_job(job))
+                };
                 let resp = ui.add(
                     egui::TextEdit::multiline(&mut self.buffer)
                         .code_editor()
-                        .font(egui::FontId::monospace(size))
                         .desired_width(f32::INFINITY)
-                        .desired_rows(30),
+                        .desired_rows(30)
+                        .layouter(&mut layouter),
                 );
                 if resp.changed() {
                     self.dirty = true;
@@ -774,9 +1228,17 @@ impl eframe::App for App {
             self.save_current();
         }
 
+        self.poll_lt();
+        self.poll_settings_test();
+
+        self.menu_bar(ctx);
         self.top_bar(ctx);
         self.left_pane(ctx);
 
+        if self.show_lt_panel {
+            self.lt_panel(ctx);
+        }
+
         if self.show_log {
             egui::TopBottomPanel::bottom("gitlog")
                 .resizable(true)
@@ -801,6 +1263,10 @@ impl eframe::App for App {
         }
 
         self.central(ctx);
+
+        if self.show_settings {
+            self.settings_window(ctx);
+        }
     }
 
     fn on_exit(&mut self, _gl: Option<&eframe::glow::Context>) {
@@ -811,6 +1277,59 @@ impl eframe::App for App {
     }
 }
 
+/// Build the editor's laid-out text, underlining any grammar/spelling matches.
+///
+/// `ranges` are `(start_byte, end_byte, is_spelling)` triples into `text`.
+/// Spelling issues get a red underline, grammar/style issues a blue one.
+fn build_editor_job(
+    ui: &egui::Ui,
+    text: &str,
+    size: f32,
+    ranges: &[(usize, usize, bool)],
+    wrap_width: f32,
+) -> egui::text::LayoutJob {
+    use egui::text::{LayoutJob, TextFormat};
+    let font_id = egui::FontId::monospace(size);
+    let color = ui.visuals().text_color();
+    let mut job = LayoutJob::default();
+    job.wrap.max_width = wrap_width;
+
+    if ranges.is_empty() {
+        job.append(text, 0.0, TextFormat::simple(font_id, color));
+        return job;
+    }
+
+    // Split the text at every match boundary, then format each run.
+    let mut points: Vec<usize> = vec![0, text.len()];
+    for &(s, e, _) in ranges {
+        points.push(s);
+        points.push(e);
+    }
+    points.retain(|&p| p <= text.len() && text.is_char_boundary(p));
+    points.sort_unstable();
+    points.dedup();
+
+    let spell_color = egui::Color32::from_rgb(0xE0, 0x40, 0x40);
+    let grammar_color = egui::Color32::from_rgb(0x3B, 0x82, 0xF6);
+    let width = (size * 0.08).max(1.5);
+
+    for w in points.windows(2) {
+        let (a, b) = (w[0], w[1]);
+        if a >= b {
+            continue;
+        }
+        let mut fmt = TextFormat::simple(font_id.clone(), color);
+        if let Some(&(_, _, spelling)) =
+            ranges.iter().find(|&&(s, e, _)| s < e && a >= s && b <= e)
+        {
+            let c = if spelling { spell_color } else { grammar_color };
+            fmt.underline = egui::Stroke::new(width, c);
+        }
+        job.append(&text[a..b], 0.0, fmt);
+    }
+    job
+}
+
 /// Very small markdown-ish preview (headings emphasised, everything else plain).
 fn render_preview(ui: &mut egui::Ui, markdown: &str) {
     use pulldown_cmark::{Event, HeadingLevel, Options, Parser, Tag, TagEnd};
@@ -887,6 +1406,21 @@ fn resolve_chapter_title(
         .unwrap_or_else(|| format!("{:0width$}.", index + 1, width = pad_width))
 }
 
+/// Re-position matches after the byte range `[s, e)` was replaced with `new_len`
+/// bytes. Matches that overlapped the edited region (including the one that was
+/// just applied) are dropped; matches entirely after it are shifted by the
+/// length delta so their highlights and offsets stay valid.
+fn remap_matches(matches: &mut Vec<crate::langtool::Match>, s: usize, e: usize, new_len: usize) {
+    matches.retain(|o| o.end <= s || o.start >= e);
+    let delta = new_len as i64 - (e - s) as i64;
+    for o in matches.iter_mut() {
+        if o.start >= e {
+            o.start = (o.start as i64 + delta) as usize;
+            o.end = (o.end as i64 + delta) as usize;
+        }
+    }
+}
+
 /// Count words in a string: runs of non-whitespace separated by whitespace.
 fn count_words(s: &str) -> usize {
     s.split_whitespace().count()
@@ -960,6 +1494,46 @@ mod tests {
         );
     }
 
+    fn m(start: usize, end: usize) -> crate::langtool::Match {
+        crate::langtool::Match {
+            start,
+            end,
+            message: String::new(),
+            replacements: Vec::new(),
+            spelling: false,
+        }
+    }
+
+    #[test]
+    fn remap_shifts_later_and_drops_overlapping() {
+        // Text "aaaa BBB cccc dddd"; replace "BBB" (5..8, len 3) with "XX" (len 2).
+        let mut matches = vec![m(0, 4), m(5, 8), m(9, 13), m(14, 18)];
+        remap_matches(&mut matches, 5, 8, 2);
+        // The applied match (5..8) is dropped; the one before is untouched;
+        // the two after shift left by 1.
+        assert_eq!(matches.len(), 3);
+        assert_eq!((matches[0].start, matches[0].end), (0, 4));
+        assert_eq!((matches[1].start, matches[1].end), (8, 12));
+        assert_eq!((matches[2].start, matches[2].end), (13, 17));
+    }
+
+    #[test]
+    fn remap_grows_when_replacement_is_longer() {
+        let mut matches = vec![m(0, 3), m(10, 14)];
+        // Replace [0,3) (len 3) with 5 bytes: delta +2 shifts the later match.
+        remap_matches(&mut matches, 0, 3, 5);
+        assert_eq!(matches.len(), 1);
+        assert_eq!((matches[0].start, matches[0].end), (12, 16));
+    }
+
+    #[test]
+    fn remap_drops_matches_overlapping_the_edit() {
+        // A match straddling the edit boundary is discarded, not mis-shifted.
+        let mut matches = vec![m(2, 7)];
+        remap_matches(&mut matches, 5, 8, 1);
+        assert!(matches.is_empty());
+    }
+
     #[test]
     fn formats_thousands_separators() {
         assert_eq!(thousands(0), "0");

src/config.rs

@@ -24,6 +24,42 @@ pub struct Config {
     /// +100 = double, -50 = half).
     #[serde(default)]
     pub editor_zoom: f32,
+    /// URL scheme for the LanguageTool server: `http` (local) or `https`.
+    #[serde(default = "default_languagetool_scheme")]
+    pub languagetool_scheme: String,
+    /// Host of the LanguageTool server — an IP address or a domain name.
+    #[serde(default = "default_languagetool_host")]
+    pub languagetool_host: String,
+    /// TCP port of the LanguageTool server.
+    #[serde(default = "default_languagetool_port")]
+    pub languagetool_port: u16,
+    /// Optional bearer token sent in the `Authorization` header (for a server
+    /// behind an auth proxy, or the premium API key). Empty = no auth.
+    #[serde(default)]
+    pub languagetool_token: String,
+    /// Language passed to LanguageTool (`auto` to detect, or a code like `en-US`).
+    #[serde(default = "default_languagetool_language")]
+    pub languagetool_language: String,
+}
+
+/// Default scheme: plain HTTP, matching a local server.
+pub fn default_languagetool_scheme() -> String {
+    "http".to_string()
+}
+
+/// Default host: the local machine.
+pub fn default_languagetool_host() -> String {
+    "localhost".to_string()
+}
+
+/// Default port: the one the standalone LanguageTool server listens on.
+pub fn default_languagetool_port() -> u16 {
+    8010
+}
+
+/// Default checking language: let the server auto-detect.
+pub fn default_languagetool_language() -> String {
+    "auto".to_string()
 }
 
 /// Default header/body separator, matching the manuscript drafting convention.
@@ -42,6 +78,11 @@ impl Default for Config {
             draft_marker: default_marker(),
             zero_pad_index: false,
             editor_zoom: 0.0,
+            languagetool_scheme: default_languagetool_scheme(),
+            languagetool_host: default_languagetool_host(),
+            languagetool_port: default_languagetool_port(),
+            languagetool_token: String::new(),
+            languagetool_language: default_languagetool_language(),
         }
     }
 }
@@ -62,6 +103,19 @@ fn config_path() -> PathBuf {
 }
 
 impl Config {
+    /// Assemble the LanguageTool base URL (`scheme://host:port`) from its parts.
+    pub fn languagetool_base_url(&self) -> String {
+        let scheme = match self.languagetool_scheme.trim() {
+            "" => "http",
+            s => s,
+        };
+        format!(
+            "{scheme}://{}:{}",
+            self.languagetool_host.trim(),
+            self.languagetool_port
+        )
+    }
+
     pub fn load() -> Config {
         let path = config_path();
         match std::fs::read_to_string(&path) {

src/langtool.rs

@@ -0,0 +1,233 @@
+//! Grammar and spelling checking via a LanguageTool HTTP server.
+//!
+//! Talks to the `/v2/check` endpoint of a LanguageTool instance (typically a
+//! local server, e.g. `http://localhost:8010`). The request is a plain
+//! `application/x-www-form-urlencoded` POST; both `http` and `https` endpoints
+//! are supported (the `ureq` dependency links a static rustls backend).
+//!
+//! LanguageTool reports match positions as UTF-16 offsets (it is a Java
+//! program). We convert those to byte offsets against the exact text we sent so
+//! callers can splice replacements and underline ranges directly in the buffer.
+
+use serde::Deserialize;
+use std::time::Duration;
+
+/// A single grammar/spelling issue, positioned as a byte range in the checked
+/// text.
+#[derive(Debug, Clone)]
+pub struct Match {
+    /// Byte offset of the issue in the checked text.
+    pub start: usize,
+    /// Byte offset one past the end of the issue.
+    pub end: usize,
+    /// Human-readable explanation of the problem.
+    pub message: String,
+    /// Suggested replacements, best first (may be empty).
+    pub replacements: Vec<String>,
+    /// True for spelling mistakes (LanguageTool category `TYPOS`), as opposed to
+    /// grammar/style issues. Used to colour the underline differently.
+    pub spelling: bool,
+}
+
+// ---- Wire format (subset of the LanguageTool JSON response) ----------------
+
+#[derive(Deserialize)]
+struct Response {
+    #[serde(default)]
+    matches: Vec<WireMatch>,
+}
+
+#[derive(Deserialize)]
+struct WireMatch {
+    message: String,
+    offset: usize,
+    length: usize,
+    #[serde(default)]
+    replacements: Vec<WireReplacement>,
+    rule: WireRule,
+}
+
+#[derive(Deserialize)]
+struct WireReplacement {
+    value: String,
+}
+
+#[derive(Deserialize)]
+struct WireRule {
+    category: WireCategory,
+}
+
+#[derive(Deserialize)]
+struct WireCategory {
+    #[serde(default)]
+    id: String,
+}
+
+/// Run a check against the LanguageTool server.
+///
+/// `server` is the base URL (e.g. `http://localhost:8010`); `language` is a
+/// LanguageTool code such as `en-US`, or `auto` to detect. A non-empty `token`
+/// is sent as an `Authorization: Bearer` header (for a server behind an auth
+/// proxy). Returned matches use byte offsets into `text`.
+pub fn check(server: &str, language: &str, token: &str, text: &str) -> Result<Vec<Match>, String> {
+    let base = server.trim().trim_end_matches('/');
+    if base.is_empty() || base.ends_with("//") {
+        return Err("No LanguageTool host set (open Settings ▸ LanguageTool)".to_string());
+    }
+    let url = format!("{base}/v2/check");
+
+    let agent = ureq::AgentBuilder::new()
+        .timeout(Duration::from_secs(30))
+        .build();
+
+    let mut request = agent.post(&url);
+    let token = token.trim();
+    if !token.is_empty() {
+        request = request.set("Authorization", &format!("Bearer {token}"));
+    }
+
+    let resp = request
+        .send_form(&[("language", language), ("text", text)])
+        .map_err(|e| friendly_error(base, e))?;
+
+    let body = resp
+        .into_string()
+        .map_err(|e| format!("Could not read LanguageTool reply: {e}"))?;
+    parse_response(text, &body)
+}
+
+/// Parse a LanguageTool `/v2/check` JSON reply, converting each match to a byte
+/// range in the `text` that was sent.
+fn parse_response(text: &str, body: &str) -> Result<Vec<Match>, String> {
+    let parsed: Response = serde_json::from_str(body)
+        .map_err(|e| format!("Unexpected reply from LanguageTool: {e}"))?;
+
+    let map = utf16_to_byte_map(text);
+    let byte_at = |u16: usize| -> usize { map.get(u16).copied().unwrap_or(text.len()) };
+
+    let matches = parsed
+        .matches
+        .into_iter()
+        .map(|m| {
+            let start = byte_at(m.offset);
+            let end = byte_at(m.offset + m.length).max(start);
+            Match {
+                start,
+                end,
+                message: m.message,
+                replacements: m.replacements.into_iter().map(|r| r.value).collect(),
+                spelling: m.rule.category.id.eq_ignore_ascii_case("TYPOS"),
+            }
+        })
+        .collect();
+    Ok(matches)
+}
+
+/// Turn a `ureq` error into a message aimed at someone running a local server.
+fn friendly_error(base: &str, err: ureq::Error) -> String {
+    match err {
+        ureq::Error::Status(code, _) => {
+            format!("LanguageTool returned HTTP {code} (is {base} a LanguageTool server?)")
+        }
+        ureq::Error::Transport(t) => {
+            format!("Could not reach LanguageTool at {base} — is the server running? ({t})")
+        }
+    }
+}
+
+/// Build a lookup from UTF-16 code-unit index to byte offset in `text`.
+///
+/// `map[i]` is the byte offset of the character boundary at UTF-16 unit `i`.
+/// Indices that land inside a surrogate pair are clamped to the next boundary.
+/// The final entry maps the end-of-string.
+fn utf16_to_byte_map(text: &str) -> Vec<usize> {
+    let mut map: Vec<usize> = Vec::with_capacity(text.len() + 1);
+    let mut units = 0usize;
+    for (byte, ch) in text.char_indices() {
+        while map.len() <= units {
+            map.push(byte);
+        }
+        units += ch.len_utf16();
+    }
+    while map.len() <= units {
+        map.push(text.len());
+    }
+    map
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn maps_ascii_offsets_one_to_one() {
+        let text = "hello world";
+        let map = utf16_to_byte_map(text);
+        assert_eq!(map[0], 0);
+        assert_eq!(map[6], 6); // 'w'
+        assert_eq!(map[11], 11); // end
+    }
+
+    #[test]
+    fn maps_multibyte_utf8_within_bmp() {
+        // "café" — 'é' is 2 bytes in UTF-8 but 1 UTF-16 unit.
+        let text = "café!";
+        let map = utf16_to_byte_map(text);
+        assert_eq!(map[0], 0); // c
+        assert_eq!(map[3], 3); // é starts at byte 3
+        assert_eq!(map[4], 5); // '!' is after the 2-byte é
+        assert_eq!(map[5], 6); // end
+    }
+
+    #[test]
+    fn maps_astral_char_as_two_units() {
+        // "a😀b": 😀 is 4 UTF-8 bytes and 2 UTF-16 units.
+        let text = "a😀b";
+        let map = utf16_to_byte_map(text);
+        assert_eq!(map[0], 0); // a
+        assert_eq!(map[1], 1); // 😀 starts at byte 1
+        assert_eq!(map[3], 5); // b, after the 4-byte emoji
+        assert_eq!(map[4], 6); // end
+    }
+
+    #[test]
+    fn parses_a_realistic_response() {
+        // Shape mirrors a real LanguageTool /v2/check reply.
+        let text = "I has a apple.";
+        let body = r#"{
+            "matches": [
+                {
+                    "message": "Did you mean 'have'?",
+                    "shortMessage": "Grammar",
+                    "offset": 2,
+                    "length": 3,
+                    "replacements": [{"value": "have"}],
+                    "rule": {"id": "HE_VERB_AGR", "category": {"id": "GRAMMAR", "name": "Grammar"}}
+                },
+                {
+                    "message": "Possible spelling mistake.",
+                    "offset": 8,
+                    "length": 5,
+                    "replacements": [{"value": "apple"}, {"value": "apples"}],
+                    "rule": {"id": "MORFOLOGIK_RULE_EN_US", "category": {"id": "TYPOS", "name": "Possible Typo"}}
+                }
+            ]
+        }"#;
+        let matches = parse_response(text, body).unwrap();
+        assert_eq!(matches.len(), 2);
+
+        assert_eq!(&text[matches[0].start..matches[0].end], "has");
+        assert_eq!(matches[0].replacements, vec!["have"]);
+        assert!(!matches[0].spelling);
+
+        assert_eq!(&text[matches[1].start..matches[1].end], "apple");
+        assert_eq!(matches[1].replacements, vec!["apple", "apples"]);
+        assert!(matches[1].spelling);
+    }
+
+    #[test]
+    fn empty_matches_is_ok() {
+        let matches = parse_response("clean text", r#"{"matches": []}"#).unwrap();
+        assert!(matches.is_empty());
+    }
+}

src/main.rs

@@ -5,6 +5,7 @@
 mod app;
 mod config;
 mod gitsync;
+mod langtool;
 mod odt;
 mod order;
 mod preprocess;