resizable: Let an application build its own resize handle

[ihavecoke]

Description

resize_handle_appearance() says it is there for "a hand-rolled handle in an application", but an application could not build one. resize_handle was #[doc(hidden)], generic over its drag payload, and took no listener beyond its own on_drag; ResizeHandleContext has no public constructor, so the appearance could not be put on anything else either.

This makes the handle a public, standalone part, the way SliderThumb is:

• resize_handle(id, axis) is public in gpui-base. gpui_component::resize_handle installs the divider appearance, as h_resizable does.
• ResizeHandle implements InteractiveElement, StatefulInteractiveElement and InteractiveElementExt. The drag goes through the standard on_drag / on_drag_move, and on_hover, on_double_click and the rest attach to the band itself. Base still owns the band, the cursor, inside(HandleEdge) and the Idle → Hovered → Pressed → Dragging progression the appearance reads.
• The band carries test_support(), so window.find(id) finds a handle.
• The hairline's color is a theme color, resizable.border (ThemeColor::resizable_border), falling back to border; the Base projection ResizableTheme::handle reads it. Left unset, nothing changes on screen.
• Resizable and the Dock move to the new builder, one call each.

Why: an application sidebar (Allsum) has a hand-written edge handle with the same three-level indicator — the same 20 / 28 / 44 px at 0.35 / 0.6 / 0.9. It needs a double click that restores the default width, a hover its hover-to-peek drawer counts as its own, and a lighter divider than border. h_resizable itself does not fit a collapsible fixed-width sidebar (it keeps panels proportional as the container resizes, and hides a panel without a transition), so the handle is the part it can share.

Public API

gpui-base

• pub fn resize_handle(id: impl Into<ElementId>, axis: Axis) -> ResizeHandle — no longer hidden; a band that resizes whatever it edges.
• pub struct ResizeHandle — now public and no longer generic.
• impl InteractiveElement for ResizeHandle, impl StatefulInteractiveElement for ResizeHandle, impl InteractiveElementExt for ResizeHandle — listeners attach to the band.

gpui-component

• pub fn resize_handle(id: impl Into<ElementId>, axis: Axis) -> ResizeHandle — the Base handle with resize_handle_appearance() installed.
• resizable::{HandleEdge, ResizeHandle} — re-exported.
• ThemeColor::resizable_border: Hsla — the divider's hairline; theme key resizable.border, optional, falls back to border.

Breaking Changes

Only the builder that was hidden:

- resize_handle::<DragPanel, DragPanel>(id, axis)
-     .on_drag(DragPanel, |drag: Rc<DragPanel>, _position, _window, cx| cx.new(|_| (*drag).clone()))
+ resize_handle(id, axis)
+     .on_drag(DragPanel, |drag: &DragPanel, _offset, _window, cx| cx.new(|_| drag.clone()))

How to Test

1. cargo test -p gpui-base --lib resizable — a_callers_listeners_reach_the_band hovers, double clicks and drags a standalone handle, and checks the container stops reading as hovered (the band occludes), the double click fires once, a drag reports moves without clicking, and the appearance still sees Idle → Hovered → Pressed → Dragging → Idle.
2. cargo test -p gpui-component --lib -- theme:: resizable dock — test_apply_config_reads_the_resizable_border covers an explicit resizable.border and the fallback; the projection test reads the new color.
3. cargo run -- resizable / cargo run -- dock: dividers look and behave as before.

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.

🤖 Generated with Claude Code