Code Editor Glitch

Just save and refresh. I haven't been able to document this problem properly, so I'll try to screen record if that ever happens to me again. It's been a problem for years.

I believe that you may have accidentally hit the insert key on keyboard. Pressing the insert key will cause your computer to replace old text with new text instead of adding onto the old text.

I really don't see why keyboards have this button since I see no practical use for it, and it can confuse people who accidentally press it.

Anyways, to fix your problem you simply have to press the insert key a second time to turn the insert mode off. The insert key should be somewhere near your backspace button depending on what keyboard you have.