Fix widow pad frames

[MDLC01]

Fixes #4234.

It turns out the empty frames created by pad have a non-zero size (they had padding applied to them). This made #2517 not work with those frames.

This PR solves the issue by applying the following transformation: each of the empty frames generated by pad that come before any content become zero-sized.¹ In case of a pad element that generates only empty frames, the last one is always kept in order to make sure the element never "disappears".

Footnotes

1. It is actually a tiny bit more complicated than that, due to some frames having negative dimensions (see this Discord message). ↩

[MDLC01]

Turns out this is harder than I thought. I'm marking this as a draft for now.

[MDLC01]

Actually, I just forgot to add a test ref! So this PR is now ready for review.

[laurmaedje]

Is there an associated issue for this?

The whole approach of skipping zero-sized frames in the flow is a bit of hack in my opinion. Adding to this with more complexity in padding seems like a potential maintainance burden to me. It is already very hard to change this in the flow already due to the various ad-hoc bug fixes in it.

[MDLC01]

Is there an associated issue for this?

There is not. I can create one.

Another idea I had after making this PR, which may feel less hacky, was to allow fragments to contain "fillers" instead of frames, which act as zero-sized frames, and are used instead of an empty frame that would have been created to fill a region when nothing fits into it. I don't know how related this would be to #4212 (if at all).

[MDLC01]

I created #4234.

[laurmaedje]

Let's wait with this until after the layout refactor (if I don't fix it myself during it). Every extra somewhat ad-hoc fix makes incremental refactoring harder.

[MDLC01]

Ok. I'm closing this PR, then.