See attached test case. Notice how it's white (in Safari, with tiled mode -- in non-tiled-mode, the first paint is OK, but subsequent paints won't be).
Grab with your mouse around 150,150 and drag around. See the screenshot for an example: the path is painted ... sometimes. Sometimes just the shadow is painted. Sometimes neither. Lots of flashes and horribly incorrect invalidation.
Applying the filter via CSS seems to work fine.
Created attachment 172695 [details]
Created attachment 172696 [details]
Oh my goodness this is messed up.
Does this bug still persist? It looks solved on Safari 11, maybe it can be closed.