This problem only occurs when the square is partly off the page, in this case a clip task is created which gets applied to the square's shadow. If the square is entirely in the page, no clip task is created and the shadow is properly rendered. As far as I can tell, this is an issue inside WebRender as there isn't a clip in the display list itself that would cause this.
Bug 1561447 Comment 5 Edit History
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This problem only occurs when the square is partly off the page, in this case a clip task is created which gets applied to the square's shadow. If the square is entirely in the page, no clip task is created and the shadow is properly rendered. As far as I can tell, this is an issue inside WebRender as there isn't a clip in the display list itself that would cause this. Also as the blur radius increases, the clip rect seems to increase as well, so we are probably taking the blur radius into account without considering the offset somewhere.
This problem only occurs when the square is partly off the page, in this case a clip task is created which gets applied to the square's shadow. If the square is entirely in the page, no clip task is created and the shadow is properly rendered. As far as I can tell, this is an issue inside WebRender as there isn't a clip in the display list itself that would cause this. Also as the blur radius increases, the clip rect seems to increase as well, so we are probably taking the blur radius into account without considering the offset.