In bug 1494422, we changed the way "initial-scale" affects the ICB size computation.
For example, consider a device width of 800px and a meta viewport tag of:
<meta name="viewport" content="width=device-width; initial-scale=2">
Prior to bug 1494422, this would size the ICB to 800 px, such that at the initial zoom level of 2, the visual viewport would be half the size of the ICB.
With bug 1494422 in place, this sizes the ICB to 400 px (so that it matches the device width at the initial scale), such that at the initial zoom level of 2 the visual viewport and the ICB coincide.
Some APZ tests were relying on the former behaviour to make the visual viewport smaller than the layout viewport. In particular, this is the case for helper_fixed_position_scroll_hittest.html; there may be others as well.
We should audit every usage of
initial-scale in an APZ test and make sure it's still doing what we want in light of bug 1494422.