scrollbar: Let the vertical scrollbar sit on the left side

[glani]

Description

The vertical scrollbar is hard-coded to the right edge and the horizontal one to the bottom. In a side-by-side diff the left pane's scrollbar belongs on its outer (left) edge, so the two panes mirror each other.

This adds ScrollbarPlacement, one value for both axes, set with one call — Scrollbar::placement, and scrollbar_placement / set_scrollbar_placement on the editor state:

	vertical	horizontal
BottomRight (default, unchanged)	right	bottom
BottomLeft	left	bottom
TopRight	right	top
TopLeft	left	top

Each axis follows its half of the placement: the track sits on that edge, the thumb and its hit area are anchored to it, and the bar slides in from that edge. When both bars show, the vertical one keeps its full height and the horizontal one stops short of it — at its start when the vertical bar is on the left, at its end when it is on the right. A single-axis scrollbar uses only its own half. The editor reserves no room for its horizontal bar at the bottom today, so at the top it overlays the first line the same way. Nothing changes by default.

Targets next: it is the first half of a mirrored side-by-side diff whose second half, #3417, builds on #3359's gutter markers, which are on next.

Screenshot

Before	After

The four placements (ScrollbarMode::Always); the story switches Dataset and Placement from one Options menu (d8057f92):

Public API

gpui_base (re-exported by gpui_component, also from gpui_component::scroll):

• ScrollbarPlacement { BottomRight, BottomLeft, TopRight, TopLeft } — default BottomRight; is_left() (vertical bar on the left), is_top() (horizontal bar at the top).
• Scrollbar::placement(mut self, placement: ScrollbarPlacement) -> Self.
• InputBaseState::scrollbar_placement(mut self, placement: ScrollbarPlacement) -> Self — builder for the editor's scrollbars.
• InputBaseState::set_scrollbar_placement(&mut self, placement: ScrollbarPlacement, cx: &mut Context<Self>) — replaces the whole placement at runtime.

How to Test

• cargo test -p gpui-base --lib scrollbar: the Scrollbar harness clicks and drags on each placement — the vertical track and thumb on the left, the horizontal track and thumb at the top, the horizontal track starting after a left bar and ending before a right one with both bars at the top, a thumb drag and a track click moving the offset the right way — and visibility_translation_moves_toward_the_nearest_edge covers all four placements on both axes; an editor layout test for TopLeft. Each fails with its code reverted.
• cargo run -p gpui-component-story -- scrollbar, then Options → Placement.

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.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

[huacnlee]

This PR adds left-side vertical scrollbars for mirrored diff panes, while preserving the existing right-side default. The requirement is reasonable, and the geometry changes appear consistent with that purpose.

Before merging, please reconsider the public placement API so it accounts for horizontal scrollbars as well. Scrollbar::side(Side) and the editor's scrollbar_side currently expose a general scrollbar setting that only describes the vertical axis. We should establish a coherent placement contract for both axes before publishing this API.

One option to consider is a single placement value representing the complete arrangement:

Scrollbar::new(&handle)
    .placement(ScrollbarPlacement::BottomLeft);

EditorState::new(window, cx)
    .scrollbar_placement(ScrollbarPlacement::BottomLeft);

For example:

• BottomRight: vertical on the right, horizontal at the bottom (the existing default).
• BottomLeft: vertical on the left, horizontal at the bottom.
• TopRight: vertical on the right, horizontal at the top.
• TopLeft: vertical on the left, horizontal at the top.

The editor's runtime setter would replace the complete placement with one call as well. For a single-axis scrollbar, only the corresponding part of the placement would apply.

This is a suggested design, not a requirement to use these exact names or this exact enum. Please use it as a reference, or propose a clearer alternative that covers both axes with one configuration method. Avoid requiring two axis-specific methods or making repeated calls to the same setter implicitly accumulate different axis settings.

The resulting implementation should consistently handle track placement, corner avoidance, thumb and hitbox geometry, and entrance direction for both axes. Please also keep the default behavior unchanged and update both documentation locales.

Validation: reviewed the diff at 0338bdc73 and the repository guides; git diff --check passed. Tests and real-window validation were not run. This request concerns the public API design, not a demonstrated regression in the current left-side geometry.

[huacnlee]

I added the Story changes locally: a single test area with an Options menu for switching Dataset and Placement, instead of displaying all four placements at once.

Please cherry-pick commit d8057f92 from the pr-3416-story-options branch into this PR:

git fetch https://github.com/longbridge/gpui-kit.git pr-3416-story-options
git cherry-pick d8057f92

For future PRs, please use a fork under your personal GitHub account and enable “Allow edits from maintainers.” This PR uses an organization-owned fork, and both SSH and HTTPS pushes to your branch were denied with permission errors despite maintainer_can_modify being enabled, so I could not apply the changes directly.

Being able to make small adjustments directly would reduce back-and-forth and help us move the PR forward faster.

[glani]

Thanks — took d8057f92 as is (fast-forward, same hash), and the description now points at the Options menu. Noted on the fork: further PRs will come from a personal fork with maintainer edits allowed.