There is same issue when moving handles (if you try to change strip length to less than 1).
I guess this could be fixed in one go, but if you don't want to do that I will accept this

source/blender/blenkernel/intern/sequencer.c
4945	Yes, it's only selected markers. Not very documented...

This revision is now accepted and ready to land.Jan 28 2020, 12:12 AM

In D6555#157790, @Richard Antalik (ISS) wrote:

There is same issue when moving handles (if you try to change strip length to less than 1).
I guess this could be fixed in one go, but if you don't want to do that I will accept this

Lets tackle that separately (since this is independent of overlap).
Moving handles has these "issues":

if you try to change strip length to less than 1 frame, marker offset will not be clamped (applySeqSlide doesnt, applySeqSlideValue doesnt, I think flushTransSeq does this, markers should follow, thats another bug, true)
(also in that case the header will happily print values that dont respect the clamping/snapping, but that can be found in other editors as well: NLA, ... -- whereas mesh snapping is always printing what is actually done, but that is a whole different [design] topic)
moving and "end" handle will also move a marker set on the "start", maybe we can be smarter here in the future (but that is also a whole different [design] topic)

I'll push this "as-is" and see what we can do about the other stuff at another time...
Thx for review btw.!

Closed by commit rBaa123555d7a7: Fix T73014: Marker sync does not work correctly if moving strip so it overlaps… (authored by Philipp Oeser (lichtwerk)). · Explain WhyJan 29 2020, 12:48 PM

This revision was automatically updated to reflect the committed changes.

Philipp Oeser (lichtwerk) added a commit: rBaa123555d7a7: Fix T73014: Marker sync does not work correctly if moving strip so it overlaps….