Fix split views on Catalyst

[bbrk24]

Draft until I can test that I didn't break it on iPad

[stackotter]

Thanks for looking into this. When testing out your changes locally I managed to make the app freeze in a resize handling update loop indefinitely by resizing the app a bunch and getting unlucky. I reproduced that behaviour a few times and couldn't on main so I think there may be some sort of feedback loop in your current layoutSubviews-based approach. I'm not exactly sure what the solution is to that.

I also noticed that the splits get allocated rather strange sizes (especially when you switch to the three column example), but I get the same behaviour on main so that doesn't necessarily need to be fixed in this PR.

[stackotter]

It seems like split views under Mac Catalyst still have issues with sizing the panes after certain types of view updates. That said, I'm happy to merge in this PR as is because it does fix resize handling under Mac Catalyst and doesn't make anything worse. If I did so I'd just rename it to make the purpose of the PR more clear.

Let me know if you'd like to tackle the initial sizing issues in this PR or if I should merge and open an issue for that to be tackled in the future.

Here are the pre-existing sizing issues I'm talking about;

Open the app (odd sizing, but not invalid):

Switch to 3 columns (first column overflows):

Select science (first and second columns overflow):

Switch to 2 columns (both columns overflow/overlap):