It seems there is a lower-bound size limit on HTML panels. I can disable all styling on a panel, and the following behavior is the same as with styling.
If I create an object in the panel, i.e. a button, and set that button's width to be 100% of the page, it works perfectly fine. But if I scale the width of the panel down to a certain limit, around 112-132 px, the button will stop scaling.
What I want to be able to do is scale the window to create a long toolbar like the sequence toolbox in Premiere Pro, or the left side panel in Photoshop.
I feel this may be a bug.
Based on this thing that I found(from 5 years ago), it seems that the issue may not even be HTML? Re: CS SDK bugs <-- Response #13 about Panel Size.
In the attached image, this is the smallest size that I can get to before the button stops resizing with the window. On the right is where I want to get to, which is a native Premiere Pro Panel.
Now, I can use some css engineering and get around it, but I'm trying to fix the root of the problem.
Thanks for your time.
Edit: I and others have confirmed that this is unique to Premiere. Photoshop does not behave this way, it resizes the contents appropriately.