1 - start a scrollbar drag 2 - keep holding mousedown 3 - mouseup outside of the browser area (e.g. the UI) The scrollbar dragging will get stuck as if you didn't lift the mouse button. I suspect this is because the mouseup event won't be forwarded to the child process if it didn't happen above the browser element (because then the browser element won't be the target for the event). I imagine we will need to do a setCapture here, and maybe at all cases of mouse events to a remote browser.

Moving to Core:XUL per https://bugzilla.mozilla.org/show_bug.cgi?id=1455336