Bug#1018026: meld: del key does not work in git dir-diff mode

[Vilius Panevėžys]

Package: meld
Version: 3.20.2-2
Severity: normal

* What led up to the situation?

0. `cd` to a Git working copy with at least one modified file.
1. Run `git difftool -d -t meld`.
2. Open a modified file.
3. Press the "delete" key on the keyboard.

* What was the outcome of this action?

The character to the left of the cursor not deleted, the following
error is printed to stderr.

Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/meld/dirdiff.py", line 1260, in on_button_delete_clicked
self.delete_selected()
File "/usr/lib/python3/dist-packages/meld/misc.py", line 54, in wrap_function
return function(args[0], pane, *args[1:], **kwargs)
File "/usr/lib/python3/dist-packages/meld/dirdiff.py", line 1024, in delete_selected
paths = self._get_selected_paths(pane)
File "/usr/lib/python3/dist-packages/meld/dirdiff.py", line 1308, in _get_selected_paths
assert pane is not None
AssertionError

* What outcome did you expect instead?

The character to the left of the cursor deleted, no errors.

When invoked with `git difftool -t meld`, "delete" key works as
expected - the problem is only reproducible in dir-diff mode.

-- System Information:
Debian Release: 11.4
APT prefers stable-security
APT policy: (500, 'stable-security'), (500, 'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 5.10.0-17-amd64 (SMP w/4 CPU threads)
Locale: LANG=en_US.utf8, LC_CTYPE=en_US.utf8 (charmap=UTF-8), LANGUAGE=en_US:en
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages meld depends on:
ii dconf-gsettings-backend [gsettings-backend] 0.38.0-2
ii gir1.2-gtksource-3.0 3.24.11-2
ii libcanberra-gtk3-module 0.30-7
ii libgtk-3-0 3.24.24-4+deb11u2
ii libgtksourceview-3.0-1 3.24.11-2
ii patch 2.7.6-7
ii python3 3.9.2-3
ii python3-cairo 1.16.2-4+b2
ii python3-gi 3.38.0-2
ii python3-gi-cairo 3.38.0-2

Versions of packages meld recommends:
ii yelp 3.38.3-1

meld suggests no packages.

-- no debconf information

[Vilius Panevėžys]

> * What outcome did you expect instead?
> The character to the left of the cursor deleted, no errors.

The expected outcome of pressing the "delete" key is to have the
character to the _right_ of the cursor deleted, of course. Sorry for
the brain fart.

[Vilius Panevėžys]

Just noticed the issue is also reproducible in the Find field: in the
scenario above between steps 2 and 3 press Ctrl-F - the same
AssertionError is produced.

[Jendrik Seipp]

It's probably best to report this to the upstream project at
https://gitlab.gnome.org/GNOME/meld/-/issues