Affected versions

• Firefox 74.0 (BuildId:20200309095159).
• Firefox 75.0b7 (BuildId:20200322132212).
• Firefox 76.0a1 (BuildId:20200322212426).

Unaffected versions

• Firefox 68.6.0esr (BuildId:20200305175243).

Affected platforms

• macOS 10.14

Unaffected platforms

• Windows 10 64bit
• Ubuntu 18.04 64bit

Preconditions

• Set the "Show scroll bars" option (from the System Preferences > General) to "Automatically based on mouse or trackpad"

Steps to reproduce

1. Launch Firefox.
2. Access the about:config page.
3. Set the devtools.responsive.browserUI.enabled pref to true.
4. Open RDM on any page.
5. Choose a device which viewport’s is taller than your browser window height (in order to enable the vertical scrollbar).
6. Hover over the “Close Responsive Design Mode” X button.

Expected result

• The RDM can be closed without any issues.

Actual result

• The vertical scorllbar is overlapping the second half of the button and RDM can be closed only by clicking the first half of the X button.

Regression Range

• I don’t think that this is a regression.

Note

• This issue seems to be reproducible only with the devtools.responsive.browserUI.enabled pref set to true

I can't reproduce this issue (and devtools.responsive.browserUI.enabled doesn't seem to exist anymore?)
Should we close this issue?

Changing to P2 since nobody is working on it atm

Hi Nicolas, sorry for the late delay here.

devtools.responsive.browserUI.enabled is no longer available per Bug 1585097 (this is now enabled by default and no longer under a pref).

I tried to reproduce this using the latest Nightly and I got mixed results on different devices. It seems that in order to reproduce this bug you have to set the "Show scroll bars" option (from the System Preferences > General) to "Automatically based on mouse or trackpad"