Nice job. There are a couple of these already out there:
Feedback:
You really need to let people use the Tab key instead of having to insert spaces manually. Even better would be to automatically start new lines at the correct indentation level, since Tab is often intercepted by browsers.
The current layout introduces ugly horizontal scroll bars when the viewport is even modestly resized, especially because the code snippets already use a fairly large font. As a result, you can’t see all the text at once. Since the program doesn’t auto-scroll to keep the cursor in view, it becomes very difficult to use unless you run it full screen.