Thunderbird doesn't show in the taskbar when called directly (Unity)

[tatokis]

Describe the bug
After hiding Thunderbird by pressing the X button, restoring it by running thunderbird in a terminal or by starting its shortcut does not make it visible in the taskbar.

To Reproduce
Steps to reproduce the behavior:

1. Start Thunderbird
2. Press the X button
3. Run thunderbird in a terminal
4. Observe Thunderbird missing from the taskbar even though it is visible

Expected behavior
Thunderbird shows in the taskbar when restored by calling it directly.

Desktop:

• OS: Linux
• Architecture (32 or 64 bit): amd64
• Desktop type / Window Manager: Unity
• Thunderbird version and arch: 1:115.6.0+build2-0ubuntu0.20.04.1 amd64
• Thunderbird package type: Ubuntu repo
• Run echo $XDG_SESSION_TYPE in a terminal and report the results: x11
• Add-on version: 0.9.8
• App version: 0.9.8
• Install type: Built from source manually
• Used Qt library: Default system repository (Using Qt version 5.12.8 in /usr/lib/x86_64-linux-gnu)

Additional context

This is the state of the window when this issue occurs:

_NET_WM_STATE(ATOM) = _NET_WM_STATE_MAXIMIZED_VERT, _NET_WM_STATE_MAXIMIZED_HORZ, _NET_WM_STATE_SKIP_TASKBAR, _NET_WM_STATE_SKIP_PAGER

whereas on initial start when it's showing up in the taskbar:

_NET_WM_STATE(ATOM) = _NET_WM_STATE_MAXIMIZED_VERT, _NET_WM_STATE_MAXIMIZED_HORZ

This used to work in the past.

[Ximi1970]

I can only replicate this problem with Unity. All the other window managers (Xfce/KDE/GNOME) do not have this problem. Also tested with several other distros / different versions.
So I think this is a bug of Unity, not a bug in SysTray-X.

[tatokis]

Thanks for taking the time and verifying that it occurs only in Unity.

Since it is an issue that still happens, I'd appreciate it if you kept the issue open until I (or someone else) had time to look into it and make a workaround. I've been swamped with things left and right, and with this being minor, I haven't had a chance to debug it.