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.