I'd actually deleted this when I moved the definition into edit.lua. However, there was a bug in the meta-driver and the deletion went to itself. And I didn't notice. And now when I actually deleted it using the meta-driver (after fixing it) the driver crashed because it deleted the definition of Line_number_width entirely from its VM image. But when I restart it is back from edit.lua.
(Why was the meta-driver only picking up deletions before the app? Some sort of race condition.)
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