Steps to reproduce:
- Click [reply] to a comment with the Reply tool. Type something. Leave your cursor in the middle of your text.
- Use the mouse/trackpad to switch to (or click on) the Source mode.
- Press Shift–Tab (should 'highlight' [put a box around] the Source tab.)
- Press the ← cursor key.
- See that it unexpectedly switches to the Visual mode and puts your cursor at the end of your text.
- Press the ← cursor key several times. See that the cursor doesn't behave normally.