Improvements to DockArea for nested Docks and styling

[landaire]

I'm stretching the "1 PR to solve 1 problem" request a bit, but unfortunately GitHub does not yet support stacked PRs across forks which would make trying to upstream these independently somewhat annoying.

Summary of changes: Adds some improvements to the DockArea to make managing docks, creating nested docks, and styling docks a bit easier.

Description

• fix(input): tolerate a window whose window_handle() panics: upstream gpui panics (unreachable!) when attempting to get a TestWindow handle. This catches the panic and gracefully recovers. I don't like the solution here, but it works for now. gpui-component does not hit this panic today because macos_accessibility is disabled in tests, but it may be required for some tests to do hit detection.
• feat(dock): expose remove_panel_id publicly on DockArea: makes DockArea::remove_panel_id public to support scenarios where you may have a panel ID but not a reference to the entity.
• fix(dock): ignore move_panel of a panel this area does not own: nested docks have a weird issue where trying to drag tabs within these nested panels could cause ghost tabs and some other strange things. This resolves that issue.
• feat(dock): expose per-node bounds for spatial overlays: during pre_paint, records leaf nodes render areas to allow for possibilities like e.g. vimimum style tab pickers (shown below).
• feat(dock): customizable tabs via render_tab, with a built-in close button: adds a PanelView::render_tab callback which allows for full customization of the Tab. Changes default behavior so that closeable tabs have a "Close Tab"-icon suffix button for closing.

Media

Screenshot of the tabs

Before	After

The DockArea::move_panel changes help allow this:

move_panels_compressed.mp4

And the overlay mentioned (in my application, but unblocked by these changes):

overlay_compressed.mp4

How to Test

Added new tests.

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]

Thanks for this PR! The tests are careful, and the move_panel ownership guard is a good catch.

I have one concern about the new close button.

In tab_panel.rs the button shows when panel.closable(cx) is true. But TabGroup::close_panel checks two more things before it closes anything:

if !self.constraints.is_closable() { return; }
if !self.draggable(cx) { return; }   // = !is_locked() && !is_last_panel()

So the X button can be drawn but do nothing when:

• the layout is locked, or
• the group is alone and has only one panel — a common start state for many apps.

This is the same trap the tab_drag comment just above already warns about:

a tab bar that forgets to ask TabGroupContext::is_draggable makes a group that has nowhere to go — a dock's last group, a locked dock — draggable anyway.

The smallest fix is to ask group.is_draggable() too. It covers both cases:

.when(group.is_draggable() && panel.closable(cx), |this| { ... })

The current tests do not catch this. drew_close_button builds a group with two panels, so is_last_panel is false, and no test uses a locked layout.

One note if you also want to cover the constraints.is_closable() gate: TabGroupContext::is_closable() is not the right call. It means "the active panel is closable", so it would hide the X on the other tabs. Base would need to expose that bit on its own.

[huacnlee]

Thanks for putting this together. I found two issues that should be addressed before merging:

1. DockArea::node_bounds can return stale bounds for removed nodes. Its public documentation says it returns None when a node is not a rendered tab group, but entries are never removed from node_bounds. After a leaf is removed, a caller retaining its NodeId can still receive an obsolete on-screen rectangle. Please clear stale entries or otherwise make the implementation match the documented contract, and add a regression test covering node removal.

2. The accessibility workaround catches panics around the entire ns_view(window) call. This can silently hide unrelated failures in handle matching or pointer conversion. Please narrow the catch_unwind scope to HasWindowHandle::window_handle, as the native input implementation does. A regression test for the expected test-window panic path would also be valuable.

There is also a testing gap in the new close button behavior: the current tests verify only that the button is rendered. Please add an interaction test that clicks the close button on a non-active tab and verifies that the correct PanelId is removed without selecting or otherwise triggering the tab click handler.

The targeted tests currently included in this PR pass locally.