Please check out r21256, it should hopefully resolve all remaining focus issues with the editor windows.

The feature is mostly working, but can behave weirdly in a few cases. Here's what I have found so far.

1. Something needs to be done to prevent the focus from getting stuck when the control it wants to return to no longer exists. This happens, for instance, when switching or replacing instruments if the new instrument/sample lacks that control. Focusing the first control seems to be the right thing to do in this case.
2. Last focus position is now restored even after switching between tabs. This is somewhat inconsistent with the behavior of lover halves (e.g. General tab/OPL editor), where it isn't restored after a tab switch. Also, resetting the focus to the first control in this case can actually be useful as a way of quickly getting back to the first control.

Please check r21267.

Everything is working great now! Thank you.

Oops. There's still one very small and really insignificant quirk found during some further testing. Focus position isn't forgotten after switching from the general tab only if the focus was in the upper half before the switch.

This should now be fixed in r21281.

Just perfect. Many thanks again.

FYI I will (at least for now) not backport these changes ot OpenMPT 1.31. Compared to the previous focus fixes, these are a lot more invasive and as we have seen during the process a lot can go wrong, so I would like this code to mature a bit more before it's thrown at regular users.