Comment 14 for bug 1721614

Created attachment 8921616
firefox-bug.mp4

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:57.0) Gecko/20100101 Firefox/57.0
Build ID: 20171024001719

Steps to reproduce:

Computer has two monitors configured (laptop's + external). Open Firefox. Then unplug the external monitor. Click on UI elements that cause a popup menu to appear (eg. right click to display context menu, click on bookmark folder, or on "burger" menu icon)

Actual results:

The popup menus appear at seemingly random locations. Moreover, the menu item highlighting do not match the actual mouse position (see attached video).
Happens on v56 and v57 beta.

Expected results:

The popup menus should appear at their correct location: right next to the mouse cursor for context menu, or next to the UI element that triggers the popup).