[Webkit-unassigned] [Bug 191545] New: [GTK][WPE] Incorrect tile coverage when resizing a layer out of the visible area

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=191545

            Bug ID: 191545
           Summary: [GTK][WPE] Incorrect tile coverage when resizing a
                    layer out of the visible area
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebKit Gtk
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: magomez at igalia.com
                CC: bugs-noreply at webkitgtk.org

I wasn't able to reproduce this on the desktop but I was able to see it when working on some low end devices.

The problem happens when resizing a layer that has a backingStore, and that layer is at that point out of the visible area. Inside TiledBackingStore, it exactly happens when the coverRect (the rect that we keep tiles for) and the visible rect don't overlap (empty intersection).
When the layer size changes, inside TiledBackingStore::createTiles, a call to resizeEdgeTiles() is required in order to resize the tiles on the edges to the new size. But as the code is now, that doesn't happen if the coverRect is empty (and in the scenario I mentioned it's emptied). As the size change is only detected inside the scope of that function and then forgotten, it's not performed when the layer comes into the visible area again, causing a glitch.

The fix would mean remembering that there was a size change so when the layer comes into the visible area again, the edge tiles are properly resized.

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