[Webkit-unassigned] [Bug 218324] New: REGRESSION(269065): [GPU Process]: Order of drawing has to be preserved when drawing a canvas to another canvas

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=218324

            Bug ID: 218324
           Summary: REGRESSION(269065): [GPU Process]: Order of drawing
                    has to be preserved when drawing a canvas to another
                    canvas
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Canvas
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: sabouhallawa at apple.com
                CC: dino at apple.com

When drawing an ImageBuffer to another ImageBuffer, the DrawingContext of the source and the destination ImageBuffers have to be flushed immediately. Otherwise an older version or a newer version of the source ImageBuffer might be drawn to the destination ImageBuffer. Consider this example and assume GPU rendering is enabled for canvas.

    <script>
        var canvas1 = document.getElementById('canvas1');
        var canvas2 = document.getElementById('canvas2');
        var canvas3 = document.getElementById('canvas3');

        var ctx3 = canvas3.getContext('2d');
        ctx3.fillStyle = 'red';
        ctx3.fillRect(0, 0, 100, 100);

        var ctx1 = canvas1.getContext('2d');
        ctx1.drawImage(canvas3, 0, 0);

        ctx3.fillStyle = 'green';
        ctx3.fillRect(0, 0, 100, 100);

        var ctx2 = canvas2.getContext('2d');
        ctx2.drawImage(canvas3, 0, 0);

        ctx3.fillStyle = 'blue';
        ctx3.fillRect(0, 0, 100, 100);
    </script>

1. If we do not flush any of the canvases after the "drawImage" calls, the first two canvases will not be drawn. This means an older version of canvas3 is drawn to canvas1 and canvas2.
2. If we flush canvas3 after the "drawImage" calls, all the canvases will be filled with blue. This means a newer version of canvas3 is drawn to canvas1 and canvas2.

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