HTML links' context menu appears in the wrong place (since Firefox 57 Quantum)

[Jani Uusitalo]

After upgrading to Quantum, the right mouse button context menu for HTML links pops up in the wrong place when the Firefox window is pushed to either side of my screen (so that the window takes up half of the horizontal space). The issue appears similar to what has been reported in bug #1721614, but for me the issue does not occur with the hamburger menu or other UI elements, only with links on webpages, and the menu highlighting behavior is different: in my issue the highlighting occurs normally only once I point the cursor at the menu.

== Steps to reproduce ==
1. Create a new profile and start Firefox using the profile.
2. Expand the window to fill the screen (if not already).
3. Enter search terms into the URL bar and hit enter to bring up a search results page.
4. Right-click a search result title to bring up the context menu. Left-click outside it to close it.
5. Grab the window title bar and push the window to the right edge of screen to resize the window to span half of screen horizontally. Release the title bar.
4. Right click the search result title again.

== What happens ==
The menu pops up on way off from where the mouse cursor is. See attached screenshot.

== What should happen ==
The context menu should pop up next to mouse cursor.

== Possible culprit ==
This seems to be somehow tied to localization: if I uninstall firefox-locale-*, then create a new Firefox profile, with this new profile the menu pops up where it should. I'm using Finnish locale, but this is reproducible with just firefox-local-en too, although the mispositioning differs between locales (Finnish pushes the menu off to the left, whereas English pushes it down and to the right).

== Workaround ==
Don't right-click on the page before resizing the window (that is, skip step #4 above). This seems to be the trigger.

== Other info ==
I'm able reproduce this in a (16.04) VM just as well as on the host desktop.

In case the collected data doesn't include this, my primary display is 2560 × 1440p, and the only display connected. xrandr output:

Screen 0: minimum 320 x 200, current 2560 x 1440, maximum 8192 x 8192
VGA-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-1 connected primary 2560x1440+0+0 (normal left inverted right x axis y axis) 527mm x 296mm
   2560x1440 59.95*+
   2048x1152 59.90
   1920x1200 59.88
   1920x1080 60.00 50.00 59.94 24.00 23.98
   1920x1080i 60.00 50.00 59.94
   1600x1200 60.00
   1680x1050 59.95
   1280x1024 75.02 60.02
   1280x800 59.81
   1152x864 75.00
   1280x720 60.00 50.00 59.94
   1024x768 75.03 60.00
   800x600 75.00 60.32
   720x576 50.00
   720x480 60.00 59.94
   640x480 75.00 60.00 59.94
   720x400 70.08
HDMI-2 disconnected (normal left inverted right x axis y axis)
HDMI-3 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: firefox 57.0+build4-0ubuntu0.16.04.5
ProcVersionSignature: Ubuntu 4.10.0-40.44~16.04.1-generic 4.10.17
Uname: Linux 4.10.0-40-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.13
Architecture: amd64
BuildID: 20171115095126
CurrentDesktop: Unity
Date: Wed Nov 22 18:35:05 2017
InstallationDate: Installed on 2016-10-13 (405 days ago)
InstallationMedia: Ubuntu-Server 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
SourcePackage: firefox
UpgradeStatus: No upgrade log present (probably fresh install)

[Jani Uusitalo (uusijani)]

Haven't seen this since upgrading to 18.04, and can't reproduce it following the instructions anymore either, so it appears to be fixed.