Checkerboarding in while scrolling a subframe when root scrollframe has overflow hidden and pinch zoomed in

STR:

1. set `apz.allow_zoom` to `true`
2. visit any bugzilla site (like this one)
3. zoom into the page and observe the left edge of the viewport

ER: content should be shown
AR: foreground content seems to disappear, looks like it's being cut off

I attached a video of the STR to show the problem a little bit better. So far, I could only reproduce this on bugzilla. Words words words words words words words words words words words words words words words words words words words words words words.