QtGraphicalEffects FastBlur seems to break Canvas line drawing

Attached is an example containing an (extracted for simplicity) FastBlur from QtGraphicalEffects, and a simple canvas example that uses a MouseArea to create strokes when the user presses and moves the mouse.

As a default, this testcase is broken.

Observe that when the FastBlur in main.qml is commented out, pressing the mouse will correctly draw things.

Observe that when (in main.qml) parenting the SketchWidget to the element with id "blurrable" as opposed to putting it into a child rectangle, pressing the mouse will correctly draw things.

I'm not certain that this problem is in the Canvas element, there seems to be some more underlying problem here, but I'm not sure what it is.