bug#55660: 29.0.50; PGTK degrades to terminal-compatible keyboard events

[Derek Upham <derek_upham@mailfence.com>]

Meaningful control keys in ASCII are @, A-Z, [ \ ] ^ and _ (0x40 through
0x5f).  VT100 terminals use the Control key to zero-out the top two bits
of seven, leaving us with 2^5=32 characters.  The above control keys
happen to be a bank of 32 that are all graphical and easy to reach.)  I
have old notes to myself that say that VT100 terminals do a little extra
mapping: Control and `?' (0x3f) sends the DEL keycode (0xff); Emacs
itself seems to map Control and `/' (0x2f) to the same keycode as
Control and `_' (0x1f)).

Window system interfaces like GTK track individual keypresses, and Emacs
supports binding other keys with Control:

  (global-set-key [(control ?<)] 'my-left-command)
  (global-set-key [(control ?>)] 'my-right-command)

Some terminals even send special character sequences to Emacs that
encode the Control modifier alongside the ">" character.  Xterm is one
of those, if you use the "xterm" terminal type.

I was experimenting with Emacs configurations (in a weekly snapshot) and
discovered that the PGTK interface does not support arbitrary Control
modifiers.  I can send any of the ASCII control keys listed above, but
for all others PGTK ignores the Control modifier.  Control-> turns into
a plain ">", for example.

These configuration options have the bug:

  --with-x-toolkit=gtk3
  --with-xwidgets=yes
  --with-pgtk=yes

These configuration options understand Control-> correctly.

  --with-x-toolkit=gtk3
  --with-xwidgets=yes
  --with-pgtk=no

I'm aware that PGTK is under active development and unreliable, and the
immediate workaround is "stop using PGTK".

Derek

-- 
Derek Upham
derek_upham@mailfence.com