Bug 109646 – Constrain fixed layers to the viewport, not the document

• Component: Layout and Rendering
• Status: RESOLVED FIXED

[Simon Fraser (smfr)]

RenderLayerBacking::updateCompositedBounds() has code to constrain composited layer positions to the document rect. This is bad for fixed-position elements, because this constrained rect will change every time the scroll offset changes. This means that whenever layout happens (which causes us to update compositing layers), we'll recompute the fixed layer bounds, and change it if we scrolled, and that will result in jiggles.

[Simon Fraser (smfr)]

Created attachment 187984 [details]
Patch

[WebKit Review Bot]

Comment on attachment 187984 [details]
Patch

Attachment 187984 [details] did not pass chromium-ews (chromium-xvfb):
Output: http://queues.webkit.org/results/16522575

New failing tests:
compositing/geometry/limit-layer-bounds-fixed.html
platform/chromium/virtual/softwarecompositing/geometry/limit-layer-bounds-fixed.html

[Benjamin Poulain]

Comment on attachment 187984 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=187984&action=review

> LayoutTests/ChangeLog:11
> +        Constrain fixed layers to the viewport, not the document
> +        https://bugs.webkit.org/show_bug.cgi?id=109646
> +
> +        Reviewed by NOBODY (OOPS!).
> +        
> +        Test with a big fixed element in a compositing layer.
> +
> +        * compositing/geometry/limit-layer-bounds-fixed-expected.txt: Added.
> +        * compositing/geometry/limit-layer-bounds-fixed.html: Added.

Wouldn't it be better to test this with a ref-test?

[Simon Fraser (smfr)]

(In reply to comment #3)
> Wouldn't it be better to test this with a ref-test?

No; the visible appearance is the unchanged (unless we ref-test the layerTreeAsText output, which is weird!).

[Benjamin Poulain]

(In reply to comment #4)
> No; the visible appearance is the unchanged (unless we ref-test the layerTreeAsText output, which is weird!).

I think I fail to see how the output verify the change. Would you mind explaining a bit how the layers differed before the change?

[Beth Dakin]

Comment on attachment 187984 [details]
Patch

Looks good! You should address Benjamin's question though.

[Simon Fraser (smfr)]

(In reply to comment #5)
> (In reply to comment #4)
> > No; the visible appearance is the unchanged (unless we ref-test the layerTreeAsText output, which is weird!).
> 
> I think I fail to see how the output verify the change. Would you mind explaining a bit how the layers differed before the change?

The
  (bounds 300.00 200.00)
for the fixed layer are different (smaller) with the patch.

[Simon Fraser (smfr)]

*** Bug 108556 has been marked as a duplicate of this bug. ***

[Simon Fraser (smfr)]

http://trac.webkit.org/changeset/143073

[Chris Dumez]

The new test appears to be failing on WK2 EFL:
http://build.webkit.org/results/EFL%20Linux%2064-bit%20Debug%20WK2/r143117%20(9569)/compositing/geometry/limit-layer-bounds-fixed-pretty-diff.html

[Simon Fraser (smfr)]

EFL might not do compositing for fixed pos. It should just get a new baseline.