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I did not refresh this before writing the above. It still applies, including deleting the commented out code once tests pass. See review.
Whoops: I forgot that using make_default means that we can (and should) delete the replaced var_changed defs. I added the one for space_num back because I mistakenly thought it was needed to make the test pass.
I will open a new issue to add a comment documenting the new TabPage(Frame) class design. The new rules will include using default var_changes when possible. Patch will include adjusting FontTab to follow the new rules.