gpui: Support hitbox blocking mouse interaction except scrolling (#31712)

tl;dr: This adds `.block_mouse_except_scroll()` which should typically
be used instead of `.occlude()` for cases when the mouse shouldn't
interact with elements drawn below an element. The rationale for
treating scroll events differently:

* Mouse move / click / styles / tooltips are for elements the user is
interacting with directly.
* Mouse scroll events are about finding the current outer scroll
container.

Most use of `occlude` should probably be switched to this, but I figured
I'd derisk this change by minimizing behavior changes to just the 3 uses
of `block_mouse_except_scroll`.

GPUI changes:

* Added `InteractiveElement::block_mouse_except_scroll()`, and removes
`stop_mouse_events_except_scroll()`

* Added `Hitbox::should_handle_scroll()` to be used when handling scroll
wheel events.

* `Window::insert_hitbox` now takes `HitboxBehavior` instead of
`occlude: bool`.

    - `false` for that bool is now `HitboxBehavior::Normal`.

    - `true` for that bool is now `HitboxBehavior::BlockMouse`.
    
    - The new mode is `HitboxBehavior::BlockMouseExceptScroll`.

* Removes `Default` impl for `HitboxId` since applications should not
manually create `HitboxId(0)`.

Release Notes:

- N/A

crates/ui/src/components/scrollbar.rs

@@ -3,9 +3,9 @@ use std::{any::Any, cell::Cell, fmt::Debug, ops::Range, rc::Rc, sync::Arc};
 use crate::{IntoElement, prelude::*, px, relative};
 use gpui::{
     Along, App, Axis as ScrollbarAxis, BorderStyle, Bounds, ContentMask, Corners, Edges, Element,
-    ElementId, Entity, EntityId, GlobalElementId, Hitbox, Hsla, IsZero, LayoutId, ListState,
-    MouseDownEvent, MouseMoveEvent, MouseUpEvent, Pixels, Point, ScrollHandle, ScrollWheelEvent,
-    Size, Style, UniformListScrollHandle, Window, quad,
+    ElementId, Entity, EntityId, GlobalElementId, Hitbox, HitboxBehavior, Hsla, IsZero, LayoutId,
+    ListState, MouseDownEvent, MouseMoveEvent, MouseUpEvent, Pixels, Point, ScrollHandle,
+    ScrollWheelEvent, Size, Style, UniformListScrollHandle, Window, quad,
 };
 
 pub struct Scrollbar {
@@ -226,7 +226,7 @@ impl Element for Scrollbar {
         _: &mut App,
     ) -> Self::PrepaintState {
         window.with_content_mask(Some(ContentMask { bounds }), |window| {
-            window.insert_hitbox(bounds, false)
+            window.insert_hitbox(bounds, HitboxBehavior::Normal)
         })
     }