Okay, I like this a lot
These are some nitpicks, but nothing too hard I don't think:
1. Syntax highlighting doesn't seem to work until after you move to the next word. eg: "5 4 store" store won't hilight until you press enter or space
2. Is it possible to have the error message for a error squiggle in the text editor show in a popup tooltip type thing if the user hovers over it?
3. View->Show line numbers doesn't seem to do anything.
4. The undo/redo stack seems too coarse to me. In Visual Studio undo won't undo more than a single word at a time, but this is set up to undo an entire sentence or paragraph at a time. This is probably more a matter of taste than anything.
5. The error window and the text in the text editor will flicker when an auto-parsing happens and tries to refresh them. I'm not sure how to prevent the flicker. You'll have to play nice with the way winforms refreshes, and that's a whole can of worms. So maybe leave this for last. But there must be a way to update a control without it flickering. You might need to google around or Stack Overflow to figure it out.
6. Save as doesn't change the name of the tab.
7. Probably just name the tab the document name and not its full path, unless possibly there's a duplicate tab with the same name open. And then maybe you show only the file path that's different and not the same. eg: robots\robotA.txt and robotsbackup\robotA.txt instead of c:\Darwinbots\robots\robotA.txt and c:\Darwinbots\robotsbackup\robotA.txt
8. It would be nice if you had something in the tab to show if the file is the same as on disk or not. At its simplest, you could do a blue circle next to the name in a tab to show that it's the same as on disk, and a red circle to show it's different. Compare what notepad++ does.
9. The find/replace window seems to flicker when it's opened for the first time. It's probably loading all the controls up fresh every time. Try creating a find/replace window at startup and just show/hide it.
10. It would be nice if F4/Shift+F4 went to the next/previous error in the error list. This is how Visual Studio does it.
11. Likewise, keep the currently selected error hilighted when you switch focus back to the text document after a user double clicks on the error.
12. Make the entire row in the error list clickable. Right now, if you try to double click anywhere right of the end of the description text for an error, nothing happens.
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I did a cursory glance at the code, but didn't take any notes. There's a few rough places, but generally it's fine. Also, when I try to double click on eg: DevControl.cs, the form designer won't show because of an error. It looks like an issue with Visual Studio and not the form itself, as the code looks fine. Does it happen for you?
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Also it exposed a bug in my DNA parsing code, so if you were trying to figure out why code like "{ 5 4 add }" was erroring out, that's my bad :/