Intent to unship: Alt-a works as "Select All" on Linux

[Masayuki Nakano]

Summary: We should stop registering `Alt-a` as a shortcut key for "Select All" on Linux.

Currently, Gecko registers `Alt-a` as a shortcut key for "Select All" on Linux (but you who use Linux may know "Ctrl-a" also work as "Select All"). However, this does not match with the other GTK applications nor Chromium, and this may cause a problem around menubar navigation. Therefore, I agreed with dropping this non-standard shortcut key.

Richard Marti (:Paenglab) and Thomas D. (:thomas8) created and tested a patch for this. Thank you.

(FYI: If your environment registers `Alt-a` as a shortcut key for "Select All", it keeps working because Gecko respects platform's behavior, this post means that Gecko's additional shortcut key definition will be dropped, and `Ctrl-a` will be added instead.)

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1376091

Standard: N/A

Platform coverage: Linux

Preference: N/A

DevTools bug: N/A

Other browsers: Chromium on Linux does not handle `Alt-a` as "Select All" on Linux

web-platform-tests: input-events/input-events-cut-paste.html assumed that `Ctrl-a` or `Meta-a` is the shortcut key for "Select All".

Security and Privacy: Nothing affects

-- 
Masayuki Nakano <masa...@d-toybox.com>
Working on DOM, Events, editor and IME handling for Gecko