I've only tested this on Windows in cmd, powershell and MINGW64 (Git bash).
In MINGW64 the arrow keys allow me to move the cursor freely to any part of the screen and begin typing there. Up and Down do NOT go through the history but rather move the cursor up/down to lines that previously had text output on them. Wherever the cursor is, I can type to that position on the screen, backspace, and press return. When pressing return the command is executed at that position on the screen, which is definitely erroneous. Again this ONLY happens in MINGW64. Other applications (cmd, powershell etc.) work as expected.
P.S. actually, testing this in VSCode terminal (which is configured to run git bash) works fine. So it's really just MINGW64's terminal window, which does not usually allow this, it seems to be only related to elm-repl.
Initial State

Cursor on wrong line, writing text

Pressing Return, command is executed at that position
