🐛 Scrollbar, disappear when the mouse is not over

[WinXaito]

Describe the bug
I'm not that sure if it's a problem with Fluent_ui or Flutter.
The scrollbar tend to disappear when the mouse move (so almost everytime I scroll, it's hard to grab the bar and it disappear).

Also, maybe it's another bug, but if I go over the scrollbar, it appear, and after that if I scroll with the mouse wheel, it stay appeared.
But wait a couple of second, the bar disappear, scroll with mouse will, you'll not see the bar again.

[bdlukaa]

I think it is an issue with Flutter's RawScrollbar, but it is resolvable using the callbacks they expose.

We hide the scrollbar as soon as the mouse leaves it, but we don't take into account if the user is still pressing it.

fluent_ui/lib/src/controls/utils/scrollbar.dart

Lines 140 to 184 in 5d756db

	@override
	void handleThumbPressStart(Offset localPosition) {
	super.handleThumbPressStart(localPosition);
	if (mounted) {
	setState(() => _dragIsActive = true);
	}
	}
	@override
	void handleThumbPressEnd(Offset localPosition, Velocity velocity) {
	super.handleThumbPressEnd(localPosition, velocity);
	if (mounted) {
	setState(() => _dragIsActive = false);
	}
	}
	@override
	void handleHover(PointerHoverEvent event) async {
	super.handleHover(event);
	// Check if the position of the pointer falls over the painted scrollbar
	if (isPointerOverScrollbar(event.position, event.kind, forHover: true)) {
	// Pointer is hovering over the scrollbar
	await contractDelay;
	if (mounted) {
	setState(() => _hoverIsActive = true);
	_hoverAnimationController.forward();
	}
	} else if (_hoverIsActive) {
	await contractDelay;
	if (mounted) {
	// Pointer was, but is no longer over painted scrollbar.
	await _hoverAnimationController.reverse();
	setState(() => _hoverIsActive = false);
	}
	}
	}
	@override
	void handleHoverExit(PointerExitEvent event) {
	super.handleHoverExit(event);
	if (mounted) {
	setState(() => _hoverIsActive = false);
	_hoverAnimationController.reverse();
	}
	}