I haven't tested but based on the current code I assume that zoom constraints do not get propagated to oop iframes, leading them to get default zoom constraints which don't allow zooming. While this doesn't affect pinch zooming because that automatically gets routed to the root content apzc this would affect double tap to zoom.

The patch https://phabricator.services.mozilla.com/D112315 from bug 1705623 would have fixed this in one way, but in bug 1705622 we decided to go a different way, which also fixes this case.

This bug is to write a test for this.

I think this is invalid now. I don't think we use the zoom constraints on iframes for anything. Do you agree?

I think the intention was to write a test for a scenario similar to the following:

• The root content document has user-scalable=no in its meta viewport tag
• There is a cross-origin iframe
• Double-tap on content inside the iframe
• The browser should respect the user-scalable=no and not zoom

We can probably add a test like that as a simple variation of helper_doubletap_zoom_oopif.html.

(I do agree that, with bug 1713715 in place, we no longer take a codepath that looks at the iframe zoom constraints, which makes the test less interesting.)

https://hg.mozilla.org/mozilla-central/rev/96c170b29323