Bug 118493 – Deferring layer flushes can cause painting without layout being done

• Status: RESOLVED
• Resolution: FIXED
• Component: Layout and Rendering

[Tim Horton]

1. We depend on a run loop observer that fires just before CA does painting to ensure that layout is up-to-date.
2. TileController manually calls setNeedsDisplay on CALayers.
3. RenderLayerCompositor can (after r147797) defer #1 and cause it to not happen in the same run loop cycle as #2.
4. This means that we can end up painting without layout being up-to-date, which can cause horrible, horrible graphical issues.

We need to not defer (#3) if TileController has setNeedsDisplay'd any layers manually (or if we've added any new dirty layers).

<rdar://problem/14286329>

[Tim Horton]

One of the best repro cases of actual real world badness caused by this is http://www.mercurynews.com, which frequently shows garbage (or black) upon loading.

[Tim Horton]

#2 is actually "TileController can parent layers which had setNeedsDisplay called on them long ago, but were unparented"

[Tim Horton]

Created attachment 206378 [details]
preliminary patch

[Tim Horton]

Created attachment 206409 [details]
patch

[Antti Koivisto]

Comment on attachment 206409 [details]
patch

For the benefit of future debuggers, it might be good to add a comment above platformCALayerDidCreateTiles call as it is not that obvious that it initiates an immediate layer flush.

[Tim Horton]

(In reply to comment #5)
> (From update of attachment 206409 [details])
> For the benefit of future debuggers, it might be good to add a comment above platformCALayerDidCreateTiles call as it is not that obvious that it initiates an immediate layer flush.

True. I'll do that.

[Tim Horton]

http://trac.webkit.org/changeset/152548