[Webkit-unassigned] [Bug 41113] New: [WinCairo] Text box backgrounds do not render in partially opaque layers

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=41113

           Summary: [WinCairo] Text box backgrounds do not render in
                    partially opaque layers
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: PC
        OS/Version: Windows XP
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: mrobinson at webkit.org
        Depends on: 41111


Even after the fixes here ( https://bugs.webkit.org/show_bug.cgi?id=41111 ), text box backgrounds do not render in partially opaque layers. The reason is that the internal Windows theme drawing code is rendering the text box background with GDI. The HDC that it is rendering to is backed by an HBITMAP with an alpha layer. Since GDI is malevolently alpha unaware, rendering onto such an HDC will peg alpha values to 0, meaning everything on the context is fully transparent. Controls other than text boxes (checkboxes and radio buttons, for instance) render in an alpha-aware fashion.

The CoreGraphics port handles this issue via a special image surface type which ignores the alpha channel. There is no such thing in Cairo, as far as I can tell, so the solution is to manually adjust the alpha values on Windows surfaces not supporting alpha-blending to 255 (fully opaque).

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