Bug 77369 – Introduce a no-repaint-expected flag

• Status: NEW
• Component: Layout and Rendering

[Julien Chaffraix]

We have been seeing some instances of over-repainting, especially with accelerated compositing enabled (see bug 50417 for example). Our current strategy to avoid regressions was to write a repaint test in those cases.

While it does work, it may miss some cases where we really shouldn't be repainting. Layout has a similar flag (RenderObject setNeedsLayoutIsForbidden) which makes me think it would be worthwhile.

[Simon Fraser (smfr)]

I don't really understand what you're trying to achieve. We generally consider repaints as cheap, as long as they get coalesced into a single paint.

[James Robinson]

Could you define your terms more precisely? What exactly do you mean by over-repainting? Too many invalidations sent out, invalidation rects that are too large, too much time calculating repaint rects, something else ... ?

[Julien Chaffraix]

> Could you define your terms more precisely? What exactly do you mean by over-repainting? Too many invalidations sent out, invalidation rects that are too large, too much time calculating repaint rects, something else ... ?

Good point, I should have said needless invalidations. With the accelerated composition, it appears that some of our invalidation could now be removed.

> I don't really understand what you're trying to achieve. We generally consider repaints as cheap, as long as they get coalesced into a single paint.

I guess that stemmed from my bad wording of the problem: the invalidation are cheap indeed, painting may not be. It's always better to avoid repainting at all if we can avoid it.

Note that I haven't investigated how difficult it would be to add the flag, just that it would be a nice addition.

[Simon Fraser (smfr)]

I don't think the layout code is written with avoiding redundant invalidations in mind. I'm not sure there's a win here.

There certainly is a win in avoiding redundant painting, but I'm not sure how you detect that.