I had a similar reaction, "weird", when I tried the isolated prompt a year or whatever ago.  I don't remember if I also changed the indent for that.  But when testing this patch, with space indents, a longer time, most of the weird feeling dissipated and I realized that I might possibly prefer this, at least sometimes.

Another reason I proposed the isolated prompt as the *current* default is that some (many?) people like to save the shell as a record of their session.  With prompts in the sidebar, saving the shell *currently* results in a text record with no prompts. (And, of course, no highlighting.)  I can read it, but I expect that most people would prefer isolated prompts to no prompts

I discussed the need for shell saving options both above and in msg392163, posted to #37904 a few hours ago.  I do not currently expect anything before next Monday.  If a followup PR, zipping the sidebar with text when saving, were prepared before then, I would be more favorable to making sidebar the current default.

I plan to ask for more opinions on idle-dev and python-list later today.