Data loss when switching among template editor tabs

[civility-bot]

Scenario:

1. Debug a template, run into an error
2. Delete the offending code block.
3. Encounter a different, earlier, error (which is odd but another issue - Is it possible to parse templates *in order*? #377), about substitution values
4. Go to the Test Data tab to check the substitution values
5. Come back to the HTML tab
6. Try to Undo the deletion of the code block from step 2.

Result: no undo. The code block is lost.

I expect a template editor with syntax highlight to behave like a code editor, and offer basic undo.