User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:26.0) Gecko/20100101 Firefox/26.0 (Beta/Release) Build ID: 20131221082715 Steps to reproduce: I'm using two screen with setup 3200x1800 on the left and 1600x1200 on the right under linux. DPI is set to 228 so by default right click menu height would only be a little bit smaller than 1200. When devPixelsPerPx is larger than 1, firefox seems to think the screen size also times the factor. Which makes window in the right screen think they are in the left screen. Actual results: in my screen resolution setup, if devPixelsPerPx is set to 1.2, after right clicking on a window in 1600x1200 screen, clicking on the left part would show menu in the first screen, while the right half click would show menu in the right screen. if devPixelsPerPx is set to some larger number (for instance 2), all popup menu that should be shown in the right screen would be shown in the left screen. Expected results: menu appears in correct screen.

It seems this bug has been fixed in Aurora.

Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:38.0) Gecko/20100101 Firefox/38.0 On the black penciled line (in the image I've attached above) is where the separation occurs for a `devPixelsPerPx` value of 1.2. I've only encountered this issue on `devPixelsPerPx` values that are between 1 and 2. It appears that this invisible line of separation changes it's x position on a linear scale based on the value of `devPixelsPerPx`

Reproduceable on Firefox 43.0 for any values of devPixelsPerPx between 1 and 2 (excluding 1.0 and 2.0).

I see this on Windows, too. It feels like it *started* happening with a multi-dpi setup but now I'm seeing it without an external monitor connected, too.

William, is this a DOM thing or a Toolkit (?) thing?

I don't see anything that indicates this is a DOM thing. My guess is that we are getting some numbers wrong in layout/xul/nsMenuPopupFrame.cpp

jfkthame, as you are recently working on Windows multi-dpi support, I suppose you have some insight around this issue.

It's possible the upcoming patches in bugs like 1240533 and 1247335 may help here. If not, I'll try to look into it further; I think there may be additional issues specific to resolution support in the Linux widget backend.

Confirming on Ubuntu 16.04, Firefox 46.0.1 X86-64.

This seem to be a duplicate of 61000