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.