o Click on the ruler, and a popup opens where you can enter text and
create a mark at the clicked time.

Hmmm. I'd recommend not doing that, because clicking (and click+dragging) on the ruler is used to seek. I'd prefer if you used a keyboard shortcut + a menu item (as suggested in the bug report). It also breaks other things (see the end of this mail).

o <Escape> deletes the mark being edited, <Enter> saves it.

OK for "Enter = save", but "Escape = delete" is wrong. It is not an expected keyboard binding. Escape should be used to "exit editing mode and discarding the changes since the last time you entered the edit mode", not deletion.

Deletion should not have a keyboard shortcut (or maybe Ctrl+Delete, but not simply "Delete" because that would prevent deleting characters with it).

All the rest sounds good so far, in theory. Good work :)

Other issues that I noticed when actually testing the patch:
- The markers are bright red, same color as the playhead. This should definitely change.
- They look a bit clunky/unpolished: they are simply rectangles instead of being pretty shapes (but I understand/presume it's not trivial to do)
- The marker labels could be shown at all times (in an ellipsized version), not requiring to always hover them with the mouse to get a tooltip)
- Clips should be able to snap to them?
- You currently can't move them/right-click them.
- Their position jumps around/breaks when you zoom in.