Issues with track pointer movement w/ keyboard

[yohannd1]

Bug Summary

In the song editor, the track pointer movement with the arrow keys seems to not be working properly.
To make matters worse, sometimes when trying to move the track, the "dotted bar" on the left of the instrument tracks disappear, and it even offsets the track pointer.

Steps for issue 1 (limited track pointer movement)

1. Open the song editor
2. Position the track pointer relatively far (e.g. 5 bars) from the start
   
3. Try to move with the arrow keys (in any direction) twice
   For both sides, the first time works, but the second time doesn't and, if you change direction, it goes not back to the original place, but to the place it would be if you first pressed the opposite direction.
   With right, or right, left:
   
   With left, or left, right:
   
   It seems that, instead of moving the pointer around, it's offsetting it by either 1 leap left or 1 leap right from the place it was positioned to start at. Playing it doesn't cause any pointer desync issues.

Steps for issue 2 (the dotted bar disappearing)

1. Put song editor in full screen (max it out in the MDI)
   
   
   Note: don't re-position the track pointer or the bug stops, and keep the song editor window focused.
2. Press the right key on the keyboard
   Before pressing:
   
   After pressing:
   
   Here, the pointer was supposed to move right by one bar, but it doesn't! Instead, the dotted bar on the left disappears and the entire content of the song (except the bar counter and the track pointer) is shifted to the left by its width, causing the track pointer to be graphically offset by the same distance.
   A more detailed example on the offset:
   
   Here, you can see offset on the right and the track pointer is positioned near the 9th bar, but, when played, the melody is from right before the 8th bar.
3. Press the left key - the dotted bar re-appears, and the bug is gone!

I think this ends up being triggered by other actions too (as I've had this issue when in other situations), but this is the first situation that I can actually reproduce consistently.

Affected LMMS versions

• This does happen in LMMS 1.3.0-alpha.1.172+ga08e7f902 (Linux x86_64, Qt 5.15.2, GCC 11.2.0)
• This does not happen in LMMS 1.2.2 (Linux x86_64, Qt 5.9.7, GCC 5.4.0 20160609) (AppImage downloaded from the releases tab)

Logs

I don't know where I can get the logs... uh.

[Monospace-V]

I can confirm that the timeline marker, when trying to control it with arrow keys, behaves very strangely. It behaves strange in both 1.2.0 and 1.3 alpha, but in different ways.

[allejok96]

@PhysSong for the record, #6338 fixes the dotted bar issue, but not the position line issue.