[Webkit-unassigned] [Bug 191577] New: [cairo] BackingStore::incorporateUpdate should use CompositeCopy to support drawsBackground=false in general Cairo ports

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=191577

            Bug ID: 191577
           Summary: [cairo] BackingStore::incorporateUpdate should use
                    CompositeCopy to support drawsBackground=false in
                    general Cairo ports
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Platform
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: Tomoki.Imai at sony.com

Created attachment 354641

  --> https://bugs.webkit.org/attachment.cgi?id=354641&action=review

Test HTML which doesn't have background color.

I'm trying to implement a transparent background feature in Cairo port, and found that it works only in GTK+ port because of BackingStore::incorporateUpdate.

# How to reproduce

The other cairo ports doesn't have transparent background feature yet, but this is reproducable in GTK+ port by removing platform specific code.

1. Delete GTK+ specific code from BackingStore::incorporateUpdate
- https://trac.webkit.org/browser/webkit/trunk/Source/WebKit/UIProcess/cairo/BackingStoreCairo.cpp?rev=227728#L96
2. Build WebKitGTK+ as usual
3. Run MiniBrowser with custom background color.
- $ Tools/Scripts/run-minibrowser --gtk --bg-color="rgba(0,0,0,0)"
4. Go to webpage which doesn't have background.
- You can use attached HTML file.

# Expected result

The webpage's background becomes transparent.

# Actual result

You see collapsed image because previous image are not cleared. (I'll attach image)

I'll post the patch.

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