Automate jumpiness test cases

[pcwalton]

It's tricky to automate them, but we should try.

For posterity, jumpiness here refers to incorrect dynamic restyling that causes fragments to jump around as you hover over them with the mouse.

[jdm]

What is it about mouse hovering that triggers the problem?

[pcwalton]

It's the most common way to effect a restyle.

I imagine that having restyle hints that allow us to avoid restyling on hover will cover up these issues too, but that'd just be a cover-up: fundamentally, these are dynamic restyling/reflow bugs.

[glennw]

It would be fairly trivial to modify the is_ready_to_paint_image_output() infrastructure to trigger and wait for a specified number of (non)incremental reflows before writing the image. Would something like this do the job?

[pcwalton]

My problem actually was that I couldn't actually trigger the problem from script at all. For some reason only hover caused the issue.

My current hypothesis is that it has something to do with the node damage.