Description of problem:

When starting Firefox 66 on Fedora 29 with the "Firefox on Wayland" .desktop entry, if a higher-DPI monitor is plugged in, Firefox won't show the following windows consistently:

* context menus
* add bookmark
* menus created by extensions
* "overflow" toolbar menu

These can be interacted with using the keyboard, but those interactions aren't visible. For example, bookmarking a page would normally pop the small, docked "add a bookmark" window. The bookmark icon in the address bar looks like it's selected, as if the window were created, and you can hit ESC on the keyboard to cancel adding a bookmark, but no window is displayed. The right-click context menu works the same way: it exists, and you can interact with it using the keyboard, but it isn't visible. Ditto with my my password manager.

Version-Release number of selected component (if applicable):

* Firefox 66 installed from  https://koji.fedoraproject.org/koji/taskinfo?taskID=33603998
* Fedora 29

Steps to Reproduce:

1. install Firefox 66
2. plug in a monitor with a lower DPI than the one you already have
3. launch with "Firefox on Wayland" using "Type to search..." or "Show Applications" launcher
4. if necessary, move Firefox to the lower DPI monitor
5. right-click to trigger a context menu, or click the bookmark button in the address bar, or open the overflow menu

Actual results:

These elements aren't visible, but can be interacted with.

Expected results:

These elements ought to be visible.

Additional info:

Split from https://bugzilla.redhat.com/show_bug.cgi?id=1679517, where the mitigation was to start Firefox using the "Firefox on Wayland" .desktop entry.