Bug#870275: Firefox: GTK port has broken keyboard navigation in the menubar

[Alex ARNAUD]

Package: firefox
Version: 54.0-2
Tags: a11y
Owner: bugs at hypra.fr
User: bugs at hypra.fr
Usertags: hypra
Forwarded: https://bugzilla.mozilla.org/show_bug.cgi?id=1166825

DESCRIPTION FROM UPSTREAM:
"Reproduction steps:

  1. Press F10
  2. Press the right arrow key several times

Actual results:

  1. This selects the first item on the File menu, File > New Tab.
  2. This ends up selecting View > Toolbars > Menubar.

Expected results:

  1. This should
      a. select the File menu on the bar (as it is the first menu on the 
menubar)
      b. [ideally] display the menu
      c. NOT selected any item on the File menu
  2. This should have selected each of the other menus in turn, 
eventually either
      a. staying on Help
     or
      b. cycling back around to File.

(#2 should just fall out of #1 for free, but I included it here to 
illustrate the problem.)

Currently, you get 1a and 1b right, but the GTK port fails on 1c. For 
comparison, the Windows port gets 1a and 1c right, but misses on the 
"nice to have" 1b; xfce4-terminal (GTK 2) and wireshark (GTK 3), whose 
menubars are hopefully typical of menubars in GTK apps, do all three. 
Each of these except for the GTK port of Firefox ends up on 2b.

Or, in tabular form:

1a  1b  1c  2b  Testcase
--  --  --  --  --------
  X   X          Firefox (GTK port)     <--- Subject of this bug report
  X       X   X  Firefox (Windows port)
  X   X   X   X  xfce4-terminal (GTK 2)
  X   X   X   X  Wireshark      (GTK 3)"