NOTE: This is python2 code. It was developed and tested with inkscape-0.92.3 -- It probably does not run with inkscape 1.0.1 and should not be needed. Inkscape 1.0.1 has the feature built-in under ...
Enable snapping with the "cusp nodes" type. Drag the segment by holding it in the corner and moving it until a cross with the text "Cusp node to cusp node" is displayed. The two segment borders should ...