feat: add opt-in trailing whitespace highlighting

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();