Well, that helps narrow down the cause. A little bit. Apparently, I infer, the problem is with Safari not honouring your resize request.
I'm pondering it. When you say "it expands a bit" do you mean it fully expands vertically but not horizontally? Since we can't see your results, we have to ask questions like this. You said "a bit" but I don't know what "a bit" means to you.
Okay! I'm able to replicate your problem! Now I can really investigate. I've noticed a pattern. When I perform the resize action three times in a row, (with a pause inserted) it always works. So if you are just looking for a quick fix, do that. If you're more interested in a perfect solution, that may take time.
I've tried it with dozens of apps, in various settings. Safari isn't the only app. The KM Editor is another such app. It always has the same behaviour as Safari in dozens of my tests. So it's probably easier to address because now we don't need a separate app (i.e., Safari) to test with. I cannot blame Safari if the same problem occurs with the KM Editor. That leaves two culprits, the KM Engine or the macOS API which is receiving the requests from the KM Engine. In my experience, when it's a choice between KM or macOS, it's usually macOS that contains the bug. I'm not sure if it's directly related, but I found a thread on a similar subject here:
I rarely tag @peternlewis because I hate to bother him, (and I'm usually wrong when I ask him a question) but I think we can ask him the following: When using the KM Editor, and "Trying" the following action, why does it take three "Tries" before the KM Editor window becomes full screen? (My versions of Safari and macOS are the same as the original poster's.)
