UB of the IDLE editor

  UB = undesirable behavior

  Undesirable behavior of the IDLE editor

  With my version of IDLE under Windows, I observe the following
  behavior:

  I assume that the editor window contains some non-empty line:

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
ccccccccccccccccccccccccccccccc|cccccccccccc
dddddddddddddddddddddddddddddddddddddddddddd

  . The cursor "|" is at the 32nd position of the third line.
  Now, I press "shift-home" and the positions up to 31 of the
  third line will be selected:

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
###############################ccccccccccccc
dddddddddddddddddddddddddddddddddddddddddddd

  ("#" means "a selected c".)

  So far, this is what I would expect.

  Now, I press "shift-right" (i.e., "shift-right arrow"
  or "shift-cursor right") to exclude the first c from the
  selection. The expected result is:

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
c##############################ccccccccccccc
dddddddddddddddddddddddddddddddddddddddddddd

  . I believe all other Windows editors behave this way, so
  I got used to it and use it to select text with as little
  keystrokes as possible.

  But with my version of IDLE, this behavior does not always occur!,
  instead sometimes, unexpectedly, text from other lines is selected
  when "shift-right" is pressed in this situation.

  I wonder if someone can understand my expectation and also
  observe the behavior of IDLE which I have described.