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)] 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. Resizable and the Dock keep their behavior; only their on_drag constructors change, to take a &T (see below).

• 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 still goes through the handle's own on_drag; on_drag_move, 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.

Why: an application sidebar 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, and a hover its hover-to-peek drawer counts as its own. 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<T: 'static, E: 'static + Render>(id: impl Into<ElementId>, axis: Axis) -> ResizeHandle<T, E> — no longer hidden, signature unchanged; a band that resizes whatever it edges.
• pub struct ResizeHandle<T, E> — now exported from the crate root.
• impl InteractiveElement, impl StatefulInteractiveElement and impl InteractiveElementExt for ResizeHandle<T, E> — listeners attach to the band. The handle's own on_drag shadows StatefulInteractiveElement::on_drag.
• pub fn on_drag(self, value: T, f: impl Fn(&T, &Point<Pixels>, &mut Window, &mut App) -> Entity<E> + 'static) -> Self — the drag carries the T itself instead of an Rc<T>, and the constructor is handed a &T the way GPUI's own on_drag is. GPUI tells drags apart by payload type, so a caller's on_drag_move::<T> never heard a drag carrying Rc<T>.

gpui-component

• pub fn resize_handle<T: 'static, E: 'static + Render>(id: impl Into<ElementId>, axis: Axis) -> ResizeHandle<T, E> — the Base handle with resize_handle_appearance() installed.
• resizable::{HandleEdge, ResizeHandle} — re-exported.

Breaking Changes

ResizeHandle::on_drag was #[doc(hidden)]; its constructor now takes &T instead of Rc<T>:

- .on_drag(DragPanel, |drag, _, _, cx| cx.new(|_| drag.deref().clone()))
+ .on_drag(DragPanel, |drag, _, _, 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 — the Dock and the divider appearance, unchanged.
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.