[Webkit-unassigned] [Bug 197783] New: Backing sharing should not happen for stacking-order descendants of already-sharing layers

Fri May 10 10:14:57 PDT 2019

https://bugs.webkit.org/show_bug.cgi?id=197783

            Bug ID: 197783
           Summary: Backing sharing should not happen for stacking-order
                    descendants of already-sharing layers
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Compositing
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: simon.fraser at apple.com

We should never do this:

S--------C-c- -- ------ ------ 0x3a6878560 (0,0) width=600 height=588 (layerID 241) {sc 28} RenderView
S----------c- -- ------ ------   + 0x3a68786b8 (0,0) width=600 height=322  <html>
-NO------CP-- -- ------ ------     n 0x3a6878810 (18,10) width=302 height=302 (layerID 246) {sc 29}  <div> class='scrollable'
S--A------p-s -- ------ ------     + 0x3a6878968 (31,31) width=200 height=200  <div> class='outer sharing'
----------p-s -- ------ ------       + 0x3a6878ac0 (30,30) width=200 height=200  <div> class='sharing'

0x3a6878968 is already sharing, so 0x3a6878ac0 should just paint into it, rather than also sharing.

This is causing assertions when trying to get painting with transforms working. We need to behave more like 0x3a6878968 was compositing.

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