Looks good! Can you please add a test for \middle . as well? Both to get code coverage and to confirm that deleting during the loop causes any issue. Should be possible to simply just add it to the new test?

I'm confused why so many other images needed to be regenerated?

It does look strange, yes. I'm unsure why the additional images were regenerated as they don't look that far off from the original but these images all use the \left and \right delimiters... maybe it's regenerating because this PR touched that part?

There are some minor changes in the spacing, although quite hard to spot. I guess this is because the width and height is computed slightly different now.

Not sure if you have tried this, but what about doing the added for-loop before determining width and height? (And keeping the old expression.) One may even move the new for-loop inside if len(middle):

Thanks for the suggestion! @oscargus

I tested moving the for-loop before the height and depth calculation, but it doesn't set the AutoHeightChar for the middle delimiters. Moving it inside the if len(middle): after the height calculation works without the additional images being regenerated.