Fix bottom panel broken on launch

[KoBeWi]

Fixes #112549

I tried different solutions too, but I don't even know what is really happening.

[lodetrick]

When working on #112792, I think I found a solution to this, it is to change the early exit condition in _repaint to:

bool panel_collapsed = get_current_tab() == -1;
if (!panel_collapsed && (get_previous_tab() != -1)) {
    return;
}

This way, repaint will now be called when both current_tab and previous_tab are -1, which will only be the case on startup. It appears that _repaint wasn't actually executing on startup due to this condition, even though we wanted it to be executed.

[Repiteo]

Superseded by #112792