[Webkit-unassigned] [Bug 194179] New: Clarify layer nomenclature for the root scrolling layers

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=194179

            Bug ID: 194179
           Summary: Clarify layer nomenclature for the root scrolling
                    layers
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Scrolling
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: simon.fraser at apple.com

We want to end up with "scroll container layer" and "scrolled contents layer" in RenderLayerCompositor.

However, on macOS, we use a different hierarchy:

                                (layer [0 55 0] [0 0 763 801] [0 0 0]
                                  (name "CALayer(0x60000040bac0) GraphicsLayer(0x1ce4fa580, 44) frame clipping")
                                  (masksToBounds true)
                                  (sublayers (array
                                    (layer [0 0 0] [0 0 0 0] [0 0 0]
                                      (name "CALayer(0x60000040bb60) GraphicsLayer(0x1ce4fa2c0, 43) frame scrolling")
                                      (sublayers (array
                                        (layer [0 0 0] [0 0 768 31377] [0 0 0]
                                          (name "CALayer(0x60000040bb20) GraphicsLayer(0x1ce4ecdc0, 41) content root")


"frame clipping" is like "scroll container". "frame scrolling" is a 0x0 layer whose position we change to effect scrolling. "content root" hosts the RenderView's layer and the document overlay root.

Scrollbar layers are siblings of the "frame clipping layer".

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