MS4.4.2 > Fretboard Diagrams — Fret Mark hot spots
The red rectangles in the attached image show the "hot spot" areas for toggling Fret Marks.
• The rectangle on the right indicates the actual and appropriate area
• The rectangle on the left extents far to the left of the fret marks.
I noticed this when clicking the Fret Number so see if that would reveal more properties. Instead clicking the Fret Number deselected the Fret Mark to the right of it.