Add mouseCursor property to RawScrollbar and ScrollbarThemeData #185733

Use case

When a multiline TextField displays a scrollbar, the mouse cursor remains as an I-beam (text cursor) when hovering over the scrollbar thumb and track. Users expect the cursor to change to a default arrow pointer when hovering over a scrollbar, matching native platform behavior on desktop and web.

The root cause is in RawScrollbarState.build() (scrollbar.dart, line 2235): the MouseRegion wrapping the scrollbar content has no cursor parameter, defaulting to MouseCursor.defer. This defers cursor resolution to the parent widget — in the case of TextField, that parent is a MouseRegion(cursor: SystemMouseCursors.text) (text_field.dart, line 1785), which covers the entire widget including the scrollbar zone.

The scrollbar has no separate hit-test target — it is painted via CustomPaint.foregroundPainter on top of the child content. Because Flutter's cursor resolution picks the first non-defer MouseRegion in hit-test order (inner to outer, mouse_cursor.dart, line 262), there is no way for consumers to override the cursor specifically for the scrollbar area without subclassing and forking the entire RawScrollbarState.build() method.

Alternatives considered:

We tested 8 workaround approaches including ScrollBehavior wrappers, custom RawScrollbar subclasses, Stack overlays, TextField.mouseCursor overrides, and direct DOM cursor manipulation (for Flutter web/CanvasKit). All fail because:

• External wrappers are outer to TextField's MouseRegion in the widget tree, so they lose cursor resolution
• Internal wrappers (via ScrollBehavior.buildScrollbar) only cover the content area, not the TextField padding zone where the scrollbar is visually rendered
• TextField.mouseCursor override changes cursor for both the text area and scrollbar — losing the I-beam for text editing

Full investigation with POC code: https://github.com/ds-vitse/spike-scrollbar-cursor-ligui-3465

No major Flutter UI library has solved this (checked: fluent_ui, macos_ui, shadcn_flutter — all extend RawScrollbar and inherit the same limitation).

Related issues: #58192 (add mouse cursor support to remaining widgets), #88371 (themable mouse cursors)

Proposal

Add a mouseCursor property to RawScrollbar and ScrollbarThemeData, similar to how other interactive widgets expose cursor customization.

Minimal change in RawScrollbarState.build() at scrollbar.dart, line 2235:

// Current:
child: MouseRegion(
  onExit: ...
  onHover: ...

// Proposed:
child: MouseRegion(
  cursor: widget.mouseCursor ?? MouseCursor.defer,
  onExit: ...
  onHover: ...

Additionally, ScrollbarThemeData should support a mouseCursor property (consistent with #88371), and the Material Scrollbar should resolve it from the theme.

An ideal implementation would use isPointerOverScrollbar() to apply the cursor only when the pointer is over the scrollbar track/thumb, not the entire content area — keeping the parent widget's cursor (e.g. I-beam for TextField) for the content area.

Flutter version: 3.35.7 (Dart 3.9.2), verified against master branch source.