What type of top level widget should I use as container for others?
Overlay, wrap box, vertical box…all they have fixed dimensions in pixels, which breaks responsiveness.
There is this “size to content” boolean but I’m not totally sure what it does.
Documentation doesn’t even mention it.
I think what you’re looking for the Canvas Panel.
Re: Size to Content, my basic understanding is that when Size to Content is true, the Widget dynamically resizes based on the content (text, image, buttons, bars) and ignores any height/width settings.
Mathew Wadstein has a short video about Sizing and he touches on this. Might be worth a watch.
While I’m not entirely sure what you mean by broken responsiveness, in the interest of optimization and best practices this article might be worth a look.