dragging timeline is broken with graphics tablet

[TheophileMot]

I have a graphics tablet with a mouse that is a stand-in for a pen (i.e., it triggers events with pointerType == 'pen') . The mouse only functions when on the surface of the tablet; when the mouse is lifted, it fires a leave event.

Here's the bug:

1. Click on a timeline and drag it.
2. With the mouse button still held down, lift it from the tablet.
3. Now the timeline no longer responds to events.

I'm not sure how the code works, but my guess is that it's getting stuck in a state where dragging == true, so further attempts to drag the timeline are aborted.

Using Chrome on a Mac.