[Webkit-unassigned] [Bug 105978] New: Coordinated Graphics: Use a device and page scale factors in WebCore.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=105978

           Summary: Coordinated Graphics: Use a device and page scale
                    factors in WebCore.
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: luxtella at company100.net


Curruntly, Coordinated Graphics manages a duplicated scale. Remove code related to a duplicated scale.

There are two major changes.
1. Don't keep scale. Instead query the scale from WebPage[Proxy].
2. CoordinatedLayerTreeHost::SetVisibleContentsRect message does not send a scale anymore. Instead reuse IPC communication of WebPage, and callback mechanism of GraphicsLayer.

FYI, note what is effectiveScaleFactor.
effectiveScaleFactor is pageScaleFactor * deviceScaleFactor.
platform webview and pageViewportController know both pageScaleFactor and deviceScaleFactor.
But CoordinatedGraphicsLayer and TiledBackingStore need to know only effectiveScaleFactor.
CoordinatedGraphicsLayer would snap pixel alignment using effectiveScaleFactor, and TiledBackingStore paints contents using effectiveScaleFactor.

However, GraphicsLayerCA snaps pixel alignment using pageScaleFactor. I think it is a potential bug, but apple uses only 2 as a deviceScaleFactor, so this potential bug does not appear yet.
In conclusion, CoordinatedGraphicsLayer and TiledBackingStore need to know final scale for device, and it is effectiveScaleFactor.

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