R1:e0289070db8f

Fix window-exiting drags and drag-and-hold behaviors in JX.Scrollbar

Summary:
Fixes two issues:

- In Firefox, dragging outside the window and releasing the mouse button would miss the `mouseup` event. This would leave the bar dragging, even though the user had released the mouse button.
- In all browsers, dragging the handle and then holding your cursor in one place for more than a second would hide the handle. Instead, never hide the handle during a drag.

Test Plan:
- In Firefox, dragged handle…