Bug 13443 – FIXME: computeAbsoluteRepaintRect() ignores control clip rects

• Component: Layout and Rendering
• Status: RESOLVED CONFIGURATION CHANGED

[Following up on bug 13391].

RenderBox::computeAbsoluteRepaintRect() ignores the lightweight clip used by some form controls, the reason being that when it's called during layout, controlClipRect() returns a wrong result. In the context of a child of the control repainting during layout, you need the control's clip rect before layout. I have considered several ways to fix this:

1) Cache the clip rects in the controls themselves post layout(). This is a very fast and simple solution, but it increases the memory footprint of controls for information that's basically needed only during layout.

2) Cache the clip rects in a hash table kept in the RenderView. Controls can cache their current clip rect when entering layout() and delete it from the hash before returning from layout().

3) Similar to 2) but use a stack, so each control pushes its clip rect onto the stack. Since controls are very rarely nested, computeAbsoluteRepaintRect() could simply walk the stack looking for |o| (no need to pass the current stack pointer when recurring).

4) Maintain a true clipping rect stack in RenderView, including the "current clip", and pushing both control clip and layer clip. This will fix bug 11489 as well.

The disadvantage of 2-4 is the complexity, which includes having to keep a "during layout" code path separate from the "after layout" code path. I'd go with 1) or 2), assuming bug 11489 will be addressed by flexbox changes.

On second thought, I like 3) better than 2).

Created attachment 14136 [details]
Controls only clip children's foreground

I had written a patch and when I wanted to test it I realized that control clip is applied only to the children's foreground. I think it's a bug, but if it isn't, then this bug is INVALID.

(In reply to comment #2)
> I think it's a bug

Bug 13453.

[Ahmad Saleem]

Created attachment 460357 [details]
All browser differs

I am not able to determine expected result across all browsers since all browser slightly differ. Chrome and Firefox are matched mostly except font rendering or text is strange and fits differently from each other in the <div>. As for Safari 15.5, it is vastly different from other browsers as shown in the attached screenshots.

Thanks!

[Radar WebKit Bug Importer]

<rdar://problem/95557957>

[Ryosuke Niwa]

Firefox & Chrome are basically matching so we're the only engine to fail this test case now.

[Ahmad Saleem]

Safari Technology Preview 164 is also matching somewhat (text related differences) with Chrome Canary 112 and Firefox Nightly 112 and don't clip overflowing stuff.

Marking this as "RESOLVED CONFIGURATION CHANGED".

Please reopen, if the text display is also part of this bug. Thanks!

Seems OK to me.