input: Add line decorations with row backgrounds and gutter markers

[glani]

Description

Editors need whole-row annotations that text styles and range decorations
cannot express: diff rows, conflicts, bookmarks, breakpoints. This PR adds
LineDecorationCollection, which is owned like TextDecorationCollection
and RangeDecorationCollection. Each collection's entries come from a
LineDecorationProvider, and the editor asks it about the visible buffer
rows on every frame it paints.

A decoration paints a background band across its row, a marker in the
line-number gutter, or both.

• Base keeps the meaning. GutterMarker is a semantic,
  #[non_exhaustive] enum: DiffAdded, DiffRemoved, DiffChanged,
  Conflict, Bookmark, Breakpoint and Custom.
• GPUI Component draws it. The styled input projects a gutter marker
  renderer, next to fold_icon_renderer, that paints a theme-colored icon
  sized from the editor's font.
• Bands paint low and wide. They sit under the active line, indent
  guides, selections and text, and continue across the gutter.
• Markers have a slot of their own. While line numbers are shown, a
  marker renderer and size are set and any collection has a provider, the
  gutter reserves a slot at the left of the numbers: the marker size plus
  a gap. The styled editor sizes the marker at 90% of its effective font
  size and the gap at 30%, so the slot follows interface zoom and an
  editor's .text_size(...). A marker never covers a digit, and the
  gutter keeps its width as marked rows scroll in and out of view.
• No interaction. Neither affects hit testing, focus or events.

Because the provider is asked again on every frame, it owns its rows and
can read colors from the theme when asked. Rows are not edit-tracked and
are not cleared by set_value. This suits annotations computed from an
external model, such as a diff or a debugger session.

As in #3040, each owner holds its own collection, so independent features
cannot replace one another's decorations. Gutter markers were out of
scope there; this PR does not add gutter interaction, lanes or an
InputEvent.

Also in this PR:

• row_bounds(row): the window-space band a laid-out buffer row occupies,
  where it is painted, including below a multi-line inline completion.
• An arrow cursor over the line-number gutter, which used to show the
  text I-beam.

Public API

gpui-base (gpui_base::input)

• #[non_exhaustive] pub enum GutterMarker { DiffAdded, DiffRemoved, DiffChanged, Conflict, Bookmark, Breakpoint, Custom { icon: SharedString, color: Hsla } }: the meaning of a gutter marker; Custom is an SVG asset path and color painted as given.
• pub struct LineDecoration: a background and/or marker for one zero-based buffer row.
  • LineDecoration::new(row: usize) -> Self: an empty decoration for a row.
  • LineDecoration::with_background(self, color: Hsla) -> Self: paint a band across the row.
  • LineDecoration::with_marker(self, marker: GutterMarker) -> Self: paint a marker in its gutter.
  • LineDecoration::row(&self) -> usize, background(&self) -> Option<Hsla>, marker(&self) -> Option<&GutterMarker>: readers.
• pub trait LineDecorationProvider { fn line_decorations(&self, rows: Range<usize>, cx: &App) -> Vec<LineDecoration>; }: supplies the decorations of the visible rows, asked every frame.
• pub struct LineDecorationCollection: the handle for one owner's provider.
  • set_provider(&self, provider: Rc<dyn LineDecorationProvider>, cx: &mut App): replace the provider.
  • clear(&self, cx: &mut App): stop painting and keep the handle.
  • dispose(&self, cx: &mut App): release the collection and invalidate its clones.
  • has_provider(&self, cx: &App) -> bool: whether there is a provider to ask.
• EditorState::create_line_decorations_collection(&mut self, provider: Rc<dyn LineDecorationProvider>, cx: &mut Context<Self>) -> LineDecorationCollection: create an independent collection.
• InputBaseState::row_bounds(&self, row: usize) -> Option<Bounds<Pixels>>: the band a laid-out buffer row occupies, gutter to right edge, across its soft wraps, moved down past inline completion ghost lines above it.
• pub type GutterMarkerRenderer = Rc<dyn Fn(&GutterMarker) -> AnyElement>: renders a marker within the size the style supplies.
• InputEditorStyle::with_gutter_marker_renderer(self, renderer: Option<GutterMarkerRenderer>) -> Self and InputEditorStyle::gutter_marker_renderer(&self) -> Option<&GutterMarkerRenderer>: the presentation seam for markers; with None, markers are not painted.
• InputEditorStyle::with_gutter_marker_size(self, size: impl Into<AbsoluteLength>) -> Self and InputEditorStyle::gutter_marker_size(&self) -> AbsoluteLength: the square marker size, resolved against the window's rem at each layout and shared by the icon, the slot and vertical centering; defaults to zero, which paints no marker and reserves no slot.
• InputEditorStyle::with_gutter_marker_gap(self, gap: impl Into<AbsoluteLength>) -> Self and InputEditorStyle::gutter_marker_gap(&self) -> AbsoluteLength: the space between a marker and the line numbers; defaults to zero.
• InputExtras::line_decorations(&self, rows: Range<usize>, cx: &App) -> Vec<LineDecoration>: defaulted; how the renderer reaches the editor's collections.
• InputExtras::has_line_decorations(&self) -> bool: defaulted to false; whether the gutter reserves the marker slot.
• #[non_exhaustive] on InputEditorStyle, whose three gutter marker fields are private: see Breaking Changes.

gpui-component (gpui_component::input)

• pub use gpui_base::input::{GutterMarker, LineDecoration, LineDecorationCollection, LineDecorationProvider};. Input/Editor project a marker renderer that uses the theme's success, danger, warning and info colors, with the marker at 90% of the editor's effective font size and the gap at 30%.

Breaking Changes

InputEditorStyle is now #[non_exhaustive], and its new gutter marker
settings are private fields behind builders. Outside gpui-base, struct
literals no longer compile, including those that end in
..Default::default(). Start from Default and assign the existing
fields to change:

-let style = InputEditorStyle {
-    foreground,
-    caret,
-    ..Default::default()
-};
+let mut style = InputEditorStyle::default();
+style.foreground = foreground;
+style.caret = caret;

Reading and assigning the existing fields is unchanged. Gutter markers are
configured through the builders; size and gap default to zero, so a Base
user supplies all three:

let style = InputEditorStyle::default()
    .with_gutter_marker_renderer(Some(renderer))
    .with_gutter_marker_size(rems(0.75))
    .with_gutter_marker_gap(rems(0.25));

gpui-component and the Base showcase are migrated in this PR.

While a line decoration collection has a provider and line numbers are
shown, the styled editor's gutter is wider by 120% of its font size (the
marker and the gap), and the text starts that much further right. Editors
without a collection are unchanged.

InputExtras gains two defaulted methods, so existing implementations
compile unchanged.

How to Test

• cargo test -p gpui-base --locked input::
• cargo test -p gpui-component --locked input::
• cargo test -p gpui-kit --features test-support --test input --locked
• cargo clippy --workspace --exclude gpui-shell --locked -- --deny warnings
• cargo run, then Editor › Decorations: the last three rows show a
  green band with +, a red band with −, and a bookmark star, in a
  slot left of the line numbers. Edit above them and the marks follow
  the text. Change Options → Font size and the markers and their slot
  follow the font. Hover the gutter and the pointer is an arrow.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (if any) is accurate.
• Passed cargo run for story tests related to the changes. (Editor › Decorations on macOS. The gutter cursor was not checked visually.)
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific): not applicable; this change is not platform-specific.

---

The line backgrounds, gutter markers, row_bounds and the gutter cursor
were first written as a fork of this repository for a merge editor and a
diff view; this PR is that work rebuilt on current main in the shape of
#3040's decoration collections. Happy to split row_bounds and the
gutter cursor into their own PRs if that is easier to review.

🤖 Generated with Claude Code

[huacnlee]

Please fix the two rendering issues below. This PR is targeted for the 0.8.0 milestone, where the InputEditorStyle API break can be handled.

For the API change, please also mark InputEditorStyle #[non_exhaustive], migrate the cross-crate struct literals in Component and the Base showcase examples, and update the Breaking Changes section. External struct literals, including those using ..Default::default(), will no longer compile once the attribute is added; document construction via Default followed by field assignments or a supported construction API.

Security: PASS for the affected scope based on source inspection. Existing CI checks passed for this commit; the rendering findings are source-derived and were not visually reproduced locally.

Please address the findings above, then request my review again.

[huacnlee]

[P2] Reserve space for gutter markers so they do not overlap line numbers. The marker starts at origin_x - editor_paddings.left and is 12px wide, but the styled code editor's left padding is at most 6px. Three-digit line numbers (for example row 100 in a 100+ line document) have no leading padding in the reserved three-digit number column, so the marker overlaps their first digit. Markers are painted after the numbers and can obscure that digit. Please allocate a dedicated marker slot and adjust the number position/gutter width accordingly. Verify marked rows with three-digit and longer line numbers, including horizontal scrolling.

[glani]

Fixed in 3f9da90. The gutter now reserves a slot of its own at the left of the numbers, the 12px marker plus a 4px gap, and the numbers and text move right by it. The slot is reserved while line numbers are shown, a marker renderer is set and a collection has a provider, so the gutter does not change width as marked rows scroll; editors without a collection are unchanged.

gutter_markers_have_a_slot_left_of_the_line_numbers covers three- and four-digit columns with the styled editor's 6px padding, before and after a horizontal scroll, and goes red without the slot. Screenshots of both columns and of the scrolled view are in the description.

[huacnlee]

[P2] Include multiline inline-completion displacement in row_bounds. This calculation sums only real buffer row heights. Text painting and layout_line_decorations add ghost_lines_height after the cursor row, so when a multiline completion is visible, row_bounds for each subsequent row returns a y coordinate above its actual rendered position by that height. For example, two extra ghost lines on row 0 make row_bounds(1) point two line heights above row 1, breaking overlays or diff connectors positioned through this new API. Please return the actual laid-out row geometry, including ghost displacement, and add regression coverage for a following row while a multiline completion is displayed.

[glani]

Fixed in 3f9da90. The ghost lines' row and height are now part of the stored layout, and row_bounds and the line decoration layout share one row walk, LastLayout::row_extents, so both place a row where its text is painted.

rows_below_a_multiline_inline_completion_are_bounded_where_painted shows two ghost lines after row 0, then checks that rows 1–3 move down by two line heights and that their backgrounds agree. It goes red with either the stored ghost lines or row_bounds' use of them taken out.

[glani]

Thanks for the review. Since 3fba497 the branch is rebased onto main at 8d8cc67, which brings in #3358, and has two new commits:

• 3f9da90: both rendering findings, answered inline.
• d7d678e: InputEditorStyle is #[non_exhaustive]. The styled input and the Base showcase now build it from Default and assign fields, and Breaking Changes shows that migration and the wider gutter.

Checked locally: cargo clippy --workspace --exclude gpui-shell --lib --bins --tests --examples --locked -- --deny warnings; cargo test --locked for -p gpui-base --lib input:: (334), -p gpui-component --lib input:: (42) and -p gpui-kit --features test-support --test input (166). On macOS I also looked at the marker slot by eye, with three- and four-digit line numbers and with horizontal scrolling.

[huacnlee]

I've implemented and squashed the remaining review fixes into one commit: 8aa3bcbc (replacing the earlier fix commits).

• Marker icons, gutter reservation and vertical centering now share presentation metrics derived from the editor's effective font size. They follow both rem-based font scaling and editor-specific .text_size(...) changes, such as the Story's Options → Font size menu.
• The new InputEditorStyle fields are private, with with_gutter_marker_renderer, with_gutter_marker_size, with_gutter_marker_gap builders and matching readers. Existing public fields retain their current API.
• Regression coverage includes rem sizes 16, 24 and 32, three- and four-digit line numbers, horizontal scrolling, and changing the editor font from 11 to 20 while keeping the window rem unchanged. English and Chinese documentation are updated.

Targeted Rust tests, 9 documentation tests, compilation, Clippy and formatting checks passed. The updated native Gallery was built and launched for testing. This is an AI-assisted implementation.

I cannot directly update this PR branch. Maintainer edits are enabled, but the fork belongs to the GigLaboCom organization. GitHub documents that the maintainer-edit permission applies to personal forks, not organization-owned forks (fork permissions). Both push attempts were rejected with HTTP 403 for my account, huacnlee.

Please cherry-pick the single squashed fix onto your PR branch:

git fetch https://github.com/longbridge/gpui-kit.git fix/3359-gutter-marker-scaling
git cherry-pick 8aa3bcbc

Fix commit

Please also update the Public API section to list the new builder/reader signatures. The styled editor sizes markers at 90% of the effective editor font size, with a gap of 30%; the slot is no longer fixed at 16px. For direct Base usage, marker size and gap default to zero and must be supplied by the presentation layer together with the renderer.

[glani]

Thanks for doing this. 8aa3bcb is on the PR branch as-is: it sat directly on d7d678e, so this was a fast-forward, with no rebase or force-push.

The description now covers it:

• Public API lists with_gutter_marker_renderer / gutter_marker_renderer(), with_gutter_marker_size / gutter_marker_size() and with_gutter_marker_gap / gutter_marker_gap(). The size and gap are AbsoluteLength and default to zero, which paints no marker and reserves no slot.
• Breaking Changes shows the builder form a Base user needs. It also replaces the fixed 16px with the styled editor's 90% / 30% of its effective font size.

Checked locally on 8aa3bcb: cargo clippy --workspace --exclude gpui-shell --lib --bins --tests --examples --locked -- --deny warnings; cargo test --locked for -p gpui-base --lib input:: (335), -p gpui-component --lib input:: (43) and -p gpui-kit --features test-support --test input (166).