Pressing ctrl+tab does not switch tabs when browser.ctrlTab.previews = true

[Brian]

In about:config set browser.ctrlTab.previews to true. Open more than two tabs so that the tab preview window will open during navigation. Press ctrl+tab.

I usually expect releasing ctrl+tab to close the preview and switch tabs.

Instead, the tab preview opens, further presses of ctrl+tab cycle through tabs, but releasing the keys does not close the preview window and change tabs. I can press escape to close the window without switching tabs, and I can click a preview to change tabs, but I can not change tabs just by pressing and releasing ctrl+tab.

I also saw this behavior in the Mozilla nightlies on this machine.

I filed a bug with Mozilla too, but I believe it is Ubuntu-specific: https://bugzilla.mozilla.org/show_bug.cgi?id=1154922

Description: Ubuntu Vivid Vervet (development branch)
Release: 15.04

firefox:
  Installed: 37.0+build2-0ubuntu1
  Candidate: 37.0+build2-0ubuntu1
  Version table:
 *** 37.0+build2-0ubuntu1 0
        500 http://us.archive.ubuntu.com/ubuntu/ vivid/main amd64 Packages
        100 /var/lib/dpkg/status

[Brian (andersrb)]

Oh, my. I have caps lock mapped to control, and this only happens when I press capslock+tab. Pressing the real ctrl+tab works as expected. Weird!

[Brian (andersrb)]

The mapping was done with gnome-tweak-tool by setting 'caps lock key behavior' to 'make caps lock an additional ctrl'.

[Paul White (paulw2u)]

Upstream bug was closed two years ago (WORKSFORME)
Comments in #1 suggest keyboard mapping issue
Closing by marking "Invalid"