selection_clear only partially clears a selection.  That it does something is proven by selection_clear.py.  It disables selection_get.  But run the file in IDLE and when it finishes, click the tk window, select a line, click Shell, and manually repeat the last 3 lines.  selection_get is disabled again, but upon clicking the title bar of the tk window (to not affect selection in the window), the selection is still highlighted.  Further experiments showed that the insertion-deletion bug of this issue remains when using selection_own and selection_clear in IDLE.

The alternative of simulating a click on the moved insert cursor is harder by works, at least on Windows.  This also triggers a status-bar update -- see #27115.