ctrl+tab switching

[Donjan Rodic <drodic phys ethz ch>]

I'm using Meld a lot recently and my productivity would be increased if 
Ctrl+Tab and Ctrl+Shift+Tab were available for switching to the next, 
respectively previous, tab (notebook page). I'm aware that GNOME doesn't 
like this particular shortcut, but I've set it up in every other GNOME 
application I use (Gedit, gnome-terminal, Nautilus) and find it to be 
highly efficient.
I've tried to hack it unsuccessfully and would like to request some 
help. This was done on Meld 3.18.0 (default Ubuntu 18.04 repo version). 
If newer versions help with this particular issue, I'll sideload... 
updates will eventually catch up.

Attempt A)
In meldwindow.py:MeldWindow:__init__() write these built-in Gtk methods:

        from meld.meldapp import app
        app.set_accels_for_action("win.tab-previous", 
["<shift><control>Tab", "<control>Page_Up"])
        app.set_accels_for_action("win.tab-next", ["<control>Tab", 
"<control>Page_Down"])
        print(app.get_accels_for_action("win.tab-next"))

The keybindings get stored (printed) correctly, but apparently Meld 
ignores this feature.


Attempt B)
In meldwindow.py:MeldWindow:__init__(), there is a block "# Add 
alternate keybindings ..." setting F5 etc. Just below, write:

        (keyval, mask) = Gtk.accelerator_parse("<Primary>J")
        accels.connect(keyval, mask, 0, lambda *_: 
self.notebook.set_current_page(1))
        (keyval, mask) = Gtk.accelerator_parse("<Primary>Tab")
        accels.connect(keyval, mask, 0, lambda *_: 
self.notebook.set_current_page(1))
        (keyval, mask) = Gtk.accelerator_parse("<Primary>KP_Tab")
        accels.connect(keyval, mask, 0, lambda *_: print("foo"))
        (keyval, mask) = Gtk.accelerator_parse("<Primary>ISO_Left_Tab")
        accels.connect(keyval, mask, 0, lambda *_: print("bar"))

Now Ctrl+J correctly switches to the second tab from the left, but 
Ctrl+Tab and Ctrl+Shift+Tab get ignored (no terminal output either). 
They do iterate through Meld's icon list at the top, for what that's worth.


Attempt C)
In filediff.py:FileDiff:on_key_event(), write:

        if event.state & Gdk.ModifierType.CONTROL_MASK and 
Gdk.keyval_name(event.keyval) == 'Tab':
            print("switch to tab right")

This correctly prints to the terminal when Ctrl+Tab is pressed. But I'm 
at a loss on how to hook it up to notebook.set_current_page() here.
app = Gtk.Application.get_default()   is empty and I can't get a higher 
parent than self.widget.


Attempt D)
In meldwindow.py:MeldWindow:__init__(), register the signal:

    self.connect("key-press-event",self.on_key_press_event)

#...

    def on_key_press_event(self, window, event):
        print("key press event")

crashes with "AttributeError: connect", unless I let MeldWindow 
additionally inherit from

  melddoc.MeldDoc

or

  GObject.GObject

and call

  melddoc.MeldDoc.__init__(self)

respectively

  GObject.Object.__init__(self)

, in which case it simply doesn't know the signal name. Also, inheriting 
from Gtk.Window yields

  File "/usr/lib/python3/dist-packages/meld/ui/gnomeglade.py", line 47, 
in __init__
      self.widget = getattr(self, root)
  RuntimeError: field is not writable


Attempt E)
Using an external library pretty much anywhere:

        from pynput.keyboard import Listener
        with Listener(on_press=self.my_press, 
on_release=self.my_release) as listener:
            listener.join()

    def my_press(key, *_):
        print("Key pressed: {0}".format(key))
    def my_release(key, *_):
        print("Key released: {0}".format(key))

Works in principle by printing any pressed keys. But it's blocking 
w.r.t. Meld's loops, so the GUI doesn't show.


Help?

Best
Donjan