As per #684#c3 the vertical marker added as fix for #770 should also snap to edges and/or sample points.
Paste from https://sigrok.org/bugzilla/show_bug.cgi?id=684#c3:
"It would be great if, when moving the individual markers as well as both markers together, they would jump from edge to edge instead of being moved continuously. Same for the cursor marker from PR770."
Note: this bug is about the vertical mouse hover line, not TimeMarker derivatives.