STR:

1. Enable WebRender
2. Enable apz.allow_zooming
3. Go to https://developer.mozilla.org/en-US/docs/Web/SVG/Element/feDropShadow
4. Pinch-zoom over the drop-shadow example
5. The drop shadow renders incorrectly but fixes itself after invalidation

It seems that to see the problem the drop shadow has to intersect the boundary of the viewport and the shadow decides that it's bounds are now limited by the viewport.

Seems to be a regression, 2020-01-01 is better, but not perfect.

@kats: can't reproduce, needs regression check

mozregression says this was fixed by https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=8ab5f308563d7e5ba91632d751895e8fc2079bd2&tochange=e2390cba7abf74b21fe9cfc20d8cfec2f1c0fcce. There's still some clipping/checkerboarding problems but that's tracked in other bugs.