feat: add opt-in trailing whitespace highlighting

TODO.md

@@ -1,4 +1,4 @@
-- [ ] add option to highlight trailing whitespace (off by default)
+- [x] add option to highlight trailing whitespace (off by default)
 - [ ] add option to put characters at certain whitespace characters (off by default)
 	- whitespace kind -> character map should be configurable.
 - [ ] github/gitlab/gitea integration (probably requries authentication with github)

docs/configuration.md

@@ -16,6 +16,7 @@ Vamp.configure do |config|
   config.git_blame = true # false disables current-line attribution
   config.git_gutter = "bar" # or "symbols", "line_numbers", "none"
   config.scroll_distance = 0 # nonnegative edge distance, or nil to center
+  config.highlight_trailing_whitespace = false # true marks line-end whitespace in red
   config.file_explorer_side = "right"
   config.file_explorer_view = "tree"
   config.file_picker_view = "tree"
@@ -52,6 +53,8 @@ Plugin paths resolve relative to the config file, not the shell's working direct
 
 The setting applies to ordinary and historical buffers, split/unified diff panes, file pickers and explorers, command/buffer palettes, completion lists, and revision selectors. Wrapped text measures visual rows, not source lines; picker distances use result rows, excluding borders, search fields, and help. Manual mouse-wheel scrolling remains detached until keyboard navigation resumes. Only `nil` or integers from `0` through the platform's Rust `usize` maximum (`18446744073709551615` on 64-bit systems) are accepted; other values reject the configuration.
 
+`config.highlight_trailing_whitespace` defaults to `false`. Set it to `true` to give trailing whitespace a red background in ordinary and historical buffers and both sides of split/unified diffs, including wrapped and horizontally scrolled text. Spaces, expanded tab cells, and Unicode whitespace at the end of a source line are highlighted; whitespace-only lines are included, but empty lines and padding beyond the text are not. Wrapping does not make interior whitespace trailing. Search, selections, and secondary carets retain their background precedence; diagnostic underlines remain visible. This changes display only, not file contents. Only literal Ruby booleans are accepted; other values reject the configuration.
+
 `config.git_blame` defaults to `true`; set it to `false` to disable current-line inline Git blame and its background worker. Only literal Ruby booleans are accepted; other values reject the configuration. Toggle it for the running session with `:git_blame.toggle`, its palette action, or a custom binding such as `config.bind "leader b", config.core.git_blame.toggle`. There is no default shortcut.
 
 `config.git_gutter` defaults to `"bar"`: a fat `▌` with one blank column between the line number and marker and another before the text. `"symbols"` uses `+` / `-` / `~` with the same spacing. Added, removed, and modified lines use green, red, and yellow respectively. `"line_numbers"` colors the existing numbers without adding a marker column; the current line retains its cyan/bold caret highlight. `"none"` hides line-gutter repository indicators, not line numbers or picker file statuses. Other modes retain diagnostic number colors independently of repository markers. Unsupported strings, empty strings, and non-string values reject the configuration.

examples/config.rb

@@ -6,6 +6,7 @@ Vamp.configure do |config|
   config.clipboard_mode = "system" # "separate" keeps ordinary y/d/c/p/P internal
   config.git_blame = true # false disables current-line inline blame
   config.scroll_distance = 0 # nonnegative edge distance, or nil to center
+  config.highlight_trailing_whitespace = false # true marks line-end whitespace in red
   config.file_explorer_side = "right"
   config.file_explorer_view = "tree"
   config.file_picker_view = "tree"

src/editor.rs

@@ -1763,6 +1763,7 @@ pub struct Editor {
     popup: PopupConfig,
     labels: BTreeMap<String, String>,
     scroll_distance: Option<usize>,
+    highlight_trailing_whitespace: bool,
     battery_charge: Option<u8>,
     battery_label: String,
     git_blame: bool,
@@ -1865,6 +1866,7 @@ impl Editor {
             popup: PopupConfig::default(),
             labels: BTreeMap::new(),
             scroll_distance: Some(0),
+            highlight_trailing_whitespace: false,
             battery_charge: None,
             battery_label: String::new(),
             git_blame: true,
@@ -2611,6 +2613,14 @@ impl Editor {
         self.scroll_distance = distance;
     }
 
+    pub(crate) fn highlight_trailing_whitespace(&self) -> bool {
+        self.highlight_trailing_whitespace
+    }
+
+    pub(crate) fn set_highlight_trailing_whitespace(&mut self, enabled: bool) {
+        self.highlight_trailing_whitespace = enabled;
+    }
+
     pub(crate) fn set_diff_context_options(&mut self, context_lines: usize, expand_lines: usize) {
         self.diff_context_options = (context_lines, expand_lines);
         if let Some(diff) = &mut self.diff {

src/ruby.rs

@@ -45,6 +45,7 @@ pub struct Config {
     pub popup: PopupConfig,
     pub labels: BTreeMap<String, String>,
     pub scroll_distance: Option<usize>,
+    pub highlight_trailing_whitespace: bool,
     pub diff_context_lines: usize,
     pub diff_expand_lines: usize,
     pub diff_view_mode: crate::diff::ViewMode,
@@ -91,6 +92,7 @@ impl Default for Config {
             popup: PopupConfig::default(),
             labels: BTreeMap::new(),
             scroll_distance: Some(0),
+            highlight_trailing_whitespace: false,
             diff_context_lines: 10,
             diff_expand_lines: 15,
             diff_view_mode: crate::diff::ViewMode::Split,
@@ -124,6 +126,7 @@ pub fn load_config(path: &Path) -> Result<Config, String> {
                 popup,
                 labels,
                 scroll_distance,
+                highlight_trailing_whitespace,
                 diff_context_lines,
                 diff_expand_lines,
                 diff_view_mode,
@@ -158,6 +161,7 @@ pub fn load_config(path: &Path) -> Result<Config, String> {
             config.popup = popup;
             config.labels = labels;
             config.scroll_distance = scroll_distance;
+            config.highlight_trailing_whitespace = highlight_trailing_whitespace;
             config.diff_context_lines = diff_context_lines;
             config.diff_expand_lines = diff_expand_lines;
             config.diff_view_mode = diff_view_mode;
@@ -491,6 +495,7 @@ struct ConfigFrame {
     labels: BTreeMap<String, String>,
     #[serde(deserialize_with = "scroll_distance")]
     scroll_distance: Option<usize>,
+    highlight_trailing_whitespace: bool,
     #[serde(deserialize_with = "diff_context_lines")]
     diff_context_lines: usize,
     #[serde(deserialize_with = "diff_expand_lines")]
@@ -1234,9 +1239,9 @@ end
             assert!(load_config(&path).unwrap_err().contains(error), "{value}");
         }
         for frame in [
-            r#"{"type":"config","version":1,"plugins":[],"lsp":{},"bindings":[],"file_explorer_side":"right","file_picker_view":"list","file_explorer_view":"tree","clipboard_mode":"system","git_blame":true,"git_gutter":"bar","scroll_distance":0,"top_bar":{},"bottom_bar":{},"popup":{"command":{},"search":{}},"labels":{},"diff_context_lines":10,"diff_expand_lines":15,"diff_view_mode":"split","diff_split_orientation":"vertical","diff_follow_working_copy":true}"#,
+            r#"{"type":"config","version":1,"plugins":[],"lsp":{},"bindings":[],"file_explorer_side":"right","file_picker_view":"list","file_explorer_view":"tree","clipboard_mode":"system","git_blame":true,"git_gutter":"bar","scroll_distance":0,"highlight_trailing_whitespace":false,"top_bar":{},"bottom_bar":{},"popup":{"command":{},"search":{}},"labels":{},"diff_context_lines":10,"diff_expand_lines":15,"diff_view_mode":"split","diff_split_orientation":"vertical","diff_follow_working_copy":true}"#,
-            r#"{"type":"config","version":1,"plugins":[],"lsp":{},"bindings":[],"leader":null,"file_explorer_side":"right","file_picker_view":"list","file_explorer_view":"tree","clipboard_mode":"system","git_blame":true,"git_gutter":"bar","scroll_distance":0,"top_bar":{},"bottom_bar":{},"popup":{"command":{},"search":{}},"labels":{},"diff_context_lines":10,"diff_expand_lines":15,"diff_view_mode":"split","diff_split_orientation":"vertical","diff_follow_working_copy":true}"#,
+            r#"{"type":"config","version":1,"plugins":[],"lsp":{},"bindings":[],"leader":null,"file_explorer_side":"right","file_picker_view":"list","file_explorer_view":"tree","clipboard_mode":"system","git_blame":true,"git_gutter":"bar","scroll_distance":0,"highlight_trailing_whitespace":false,"top_bar":{},"bottom_bar":{},"popup":{"command":{},"search":{}},"labels":{},"diff_context_lines":10,"diff_expand_lines":15,"diff_view_mode":"split","diff_split_orientation":"vertical","diff_follow_working_copy":true}"#,
-            r#"{"type":"config","version":1,"plugins":[],"lsp":{},"bindings":[],"leader":7,"file_explorer_side":"right","file_picker_view":"list","file_explorer_view":"tree","clipboard_mode":"system","git_blame":true,"git_gutter":"bar","scroll_distance":0,"top_bar":{},"bottom_bar":{},"popup":{"command":{},"search":{}},"labels":{},"diff_context_lines":10,"diff_expand_lines":15,"diff_view_mode":"split","diff_split_orientation":"vertical","diff_follow_working_copy":true}"#,
+            r#"{"type":"config","version":1,"plugins":[],"lsp":{},"bindings":[],"leader":7,"file_explorer_side":"right","file_picker_view":"list","file_explorer_view":"tree","clipboard_mode":"system","git_blame":true,"git_gutter":"bar","scroll_distance":0,"highlight_trailing_whitespace":false,"top_bar":{},"bottom_bar":{},"popup":{"command":{},"search":{}},"labels":{},"diff_context_lines":10,"diff_expand_lines":15,"diff_view_mode":"split","diff_split_orientation":"vertical","diff_follow_working_copy":true}"#,
         ] {
             assert!(serde_json::from_str::<Response>(frame).is_err(), "{frame}");
         }
@@ -1313,6 +1318,7 @@ end
             "file_explorer_view": "tree", "clipboard_mode": "system", "git_blame": true,
             "git_gutter": "bar",
             "scroll_distance": 0,
+            "highlight_trailing_whitespace": false,
             "top_bar": {}, "bottom_bar": {}, "popup": {"command": {}, "search": {}},
             "labels": {}, "diff_context_lines": 10, "diff_expand_lines": 15,
             "diff_view_mode": "split", "diff_split_orientation": "vertical",
@@ -1331,6 +1337,22 @@ end
         assert!(serde_json::from_value::<Response>(frame).is_err());
     }
 
+    #[test]
+    fn trailing_whitespace_configuration_rejects_non_booleans() {
+        let fixture = Fixture::new();
+        for value in ["nil", "0", "\"true\"", "[]", "{}"] {
+            let path = fixture.file(
+                "config.rb",
+                &format!("Vamp.configure {{ |c| c.highlight_trailing_whitespace = {value} }}"),
+            );
+            assert!(
+                load_config(&path)
+                    .unwrap_err()
+                    .contains("highlight_trailing_whitespace must be true or false")
+            );
+        }
+    }
+
     #[test]
     fn working_diff_tracking_rejects_non_boolean_configuration() {
         let fixture = Fixture::new();
@@ -1564,6 +1586,7 @@ end
             "file_picker_view": "tree", "file_explorer_view": "tree",
             "top_bar": {}, "bottom_bar": {}, "labels": {},
             "scroll_distance": 0,
+            "highlight_trailing_whitespace": false,
             "popup": {"command": {}, "search": {}},
             "clipboard_mode": "system", "diff_context_lines": 0, "diff_expand_lines": 7,
             "diff_view_mode": "split", "diff_split_orientation": "vertical",
@@ -1612,6 +1635,7 @@ end
             "bindings": [], "leader": "space", "file_explorer_side": "right",
             "file_picker_view": "tree", "file_explorer_view": "tree",
             "clipboard_mode": "system", "git_blame": true, "git_gutter": "bar",
+            "highlight_trailing_whitespace": false,
             "top_bar": {}, "bottom_bar": {}, "labels": {},
             "popup": {"command": {}, "search": {}},
             "diff_context_lines": 10, "diff_expand_lines": 15,
@@ -2040,6 +2064,7 @@ end"#,
             "git_blame": true,
             "git_gutter": "bar",
             "scroll_distance": 0,
+            "highlight_trailing_whitespace": false,
             "top_bar": {}, "bottom_bar": {}, "labels": {},
             "popup": {"command": {}, "search": {}},
             "clipboard_mode": "system", "diff_context_lines": 10, "diff_expand_lines": 15,

src/ruby_config.rb

@@ -142,6 +142,7 @@ module Vamp
       @popup = PopupConfig.new
       @labels = {}
       @scroll_distance = 0
+      @highlight_trailing_whitespace = false
       @diff_context_lines = 10
       @diff_expand_lines = 15
       @diff_view_mode = "split"
@@ -221,6 +222,14 @@ module Vamp
       @scroll_distance = distance.nil? ? nil : diff_line_count!(distance, "scroll_distance", 0)
     end
 
+    # @param enabled [Boolean]
+    # @return [Boolean]
+    def highlight_trailing_whitespace=(enabled)
+      raise TypeError, "highlight_trailing_whitespace must be true or false" unless [true, false].include?(enabled)
+
+      @highlight_trailing_whitespace = enabled
+    end
+
     # @param lines [Integer] nonnegative context lines around each diff change
     # @return [Integer]
     def diff_context_lines=(lines)
@@ -293,6 +302,7 @@ module Vamp
         "git_blame" => @git_blame,
         "git_gutter" => @git_gutter,
         "scroll_distance" => @scroll_distance,
+        "highlight_trailing_whitespace" => @highlight_trailing_whitespace,
         "top_bar" => @bar.top, "bottom_bar" => @bar.bottom, "labels" => @labels,
         "popup" => { "command" => @popup.command, "search" => @popup.search },
         "diff_context_lines" => @diff_context_lines, "diff_expand_lines" => @diff_expand_lines,

src/session.rs

@@ -257,6 +257,7 @@ impl EditorSession {
         editor.set_git_blame(config.git_blame);
         editor.set_gutter_style(config.git_gutter);
         editor.set_scroll_distance(config.scroll_distance);
+        editor.set_highlight_trailing_whitespace(config.highlight_trailing_whitespace);
         editor.set_diff_context_options(config.diff_context_lines, config.diff_expand_lines);
         editor.set_diff_display_options(config.diff_view_mode, config.diff_split_orientation);
         if let Err(error) =

src/tui.rs

@@ -1865,6 +1865,9 @@ fn draw_document(
                 area,
                 segment.start,
             );
+            if editor.highlight_trailing_whitespace() {
+                paint_trailing_whitespace(frame, line, area, segment.start);
+            }
             paint_editor_line(
                 frame,
                 editor,
@@ -2212,6 +2215,21 @@ fn paint_cells(frame: &mut Frame, area: Rect, left: usize, start: usize, end: us
     }
 }
 
+fn paint_trailing_whitespace(frame: &mut Frame, line: &str, area: Rect, left: usize) {
+    let prefix = line.trim_end_matches(char::is_whitespace);
+    if prefix.len() == line.len() {
+        return;
+    }
+    paint_cells(
+        frame,
+        area,
+        left,
+        display_column(prefix, usize::MAX),
+        display_column(line, usize::MAX),
+        Style::default().bg(Color::Red),
+    );
+}
+
 fn paint_grapheme_range(
     frame: &mut Frame,
     area: Rect,
@@ -2919,6 +2937,7 @@ fn draw_diff(
                     ),
                     wrapped,
                     panels.diff_colors,
+                    editor.is_some_and(Editor::highlight_trailing_whitespace),
                 );
                 text_widths[0] = text_area.width;
                 if let Some(editor) =
@@ -2955,6 +2974,7 @@ fn draw_diff(
                     ),
                     wrapped,
                     panels.diff_colors,
+                    editor.is_some_and(Editor::highlight_trailing_whitespace),
                 );
                 text_widths[1] = text_area.width;
                 if let Some(editor) =
@@ -3109,6 +3129,7 @@ fn draw_diff_pane(
     viewport: (usize, &mut usize, Option<u16>),
     wrapped: Option<&[WrappedDiffRow]>,
     colors: DiffColors,
+    highlight_trailing_whitespace: bool,
 ) -> Rect {
     if area.width == 0 || area.height == 0 {
         return area;
@@ -3281,6 +3302,9 @@ fn draw_diff_pane(
                 }
             }
         }
+        if highlight_trailing_whitespace {
+            paint_trailing_whitespace(frame, &text, text_area, left);
+        }
     }
     Rect::new(
         content.x + gutter as u16,
@@ -4859,6 +4883,107 @@ mod tests {
         }
     }
 
+    #[test]
+    fn trailing_whitespace_is_opt_in_and_preserves_wrapping_and_selection() {
+        let directory = Directory::new();
+        let source = "a b\t \u{3000}\n \t\n\nabcdefg  ";
+        let path = directory.file("whitespace.txt", source);
+        let config = directory.file(
+            "config.rb",
+            "Vamp.configure { |c| c.highlight_trailing_whitespace = true; c.git_gutter = 'none' }",
+        );
+        let mut session = EditorSession::open(Some(path), vec![], Some(&config)).unwrap();
+        session.execute("word_wrap.toggle");
+        let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(10, 10)).unwrap();
+        let mut views = BufferViewports::default();
+        terminal
+            .draw(|frame| views.draw(frame, session.editor_mut()))
+            .unwrap();
+        let buffer = terminal.backend().buffer();
+        for (y, range) in [(0, 5..9), (1, 2..6), (3, 9..10), (4, 2..3)] {
+            for x in 2..10 {
+                // The backend skips the covered cell of a wide grapheme.
+                if (x, y) == (8, 0) {
+                    continue;
+                }
+                assert_eq!(
+                    buffer[(x, y)].bg == Color::Red,
+                    range.contains(&x),
+                    "({x}, {y})"
+                );
+            }
+        }
+        assert_eq!(buffer[(2, 2)].bg, Color::Reset);
+        session.handle(Input::End);
+        session.handle(Input::Char('v'));
+        session.handle(Input::Left);
+        terminal
+            .draw(|frame| views.draw(frame, session.editor_mut()))
+            .unwrap();
+        assert_eq!(terminal.backend().buffer()[(6, 0)].bg, Color::Blue);
+        assert_eq!(session.editor().lines().join("\n"), source);
+        assert!(!session.editor().dirty());
+
+        let mut editor = Editor::open(None, vec![]).unwrap();
+        editor
+            .apply(vec![crate::editor::Effect::Replace("x \t".into())])
+            .unwrap();
+        terminal
+            .draw(|frame| views.draw(frame, &mut editor))
+            .unwrap();
+        assert_ne!(terminal.backend().buffer()[(7, 0)].bg, Color::Red);
+    }
+
+    #[test]
+    fn trailing_whitespace_covers_both_diff_sides_and_unified_rows() {
+        for mode in [ViewMode::Split, ViewMode::Unified] {
+            for wrapped in [false, true] {
+                let mut editor = Editor::open(None, vec![]).unwrap();
+                editor.set_diff_display_options(mode, crate::panes::SplitAxis::Vertical);
+                install_readonly_diff(
+                    &mut editor,
+                    syntax_diff_file("whitespace.txt", "old \t", "new \t"),
+                );
+                editor.set_highlight_trailing_whitespace(true);
+                if wrapped {
+                    editor.execute("word_wrap.toggle");
+                }
+                let mut terminal =
+                    Terminal::new(ratatui::backend::TestBackend::new(80, 15)).unwrap();
+                let mut views = BufferViewports::default();
+                render_native_diff(&mut terminal, &mut views, &mut editor);
+                let buffer = terminal.backend().buffer();
+                for word in ["old", "new"] {
+                    let start = buffer
+                        .content
+                        .iter()
+                        .enumerate()
+                        .find_map(|(index, cell)| {
+                            (cell.symbol() == &word[..1]
+                                && buffer
+                                    .content
+                                    .get(index + 1)
+                                    .is_some_and(|cell| cell.symbol() == &word[1..2])
+                                && buffer
+                                    .content
+                                    .get(index + 2)
+                                    .is_some_and(|cell| cell.symbol() == &word[2..3]))
+                            .then_some(index)
+                        })
+                        .unwrap();
+                    for offset in 3..8 {
+                        assert_eq!(
+                            buffer.content[start + offset].bg,
+                            Color::Red,
+                            "{mode:?} {wrapped} {word}"
+                        );
+                    }
+                    assert_ne!(buffer.content[start + 8].bg, Color::Red);
+                }
+            }
+        }
+    }
+
     #[test]
     fn scroll_distance_follows_document_and_diff_visual_rows_and_columns() {
         let directory = Directory::new();