[Ultra App Kit and widget layouts.]

Does that event happen on the initial layout?

[Ultra App Kit and widget layouts.]

Yes I have watched the video. The "Bottom Panel" gives me the parent arrangement I want, but now I have to go in an manually calculate the button positions within that panel, and keep that up to date as the UI is resized.

I think I see how I can accomplish what I am after. If I subclass Panel, I can create a HorizontalPanel and VerticalPanel that will recenter the content on a call to UpdateLayout(?).

[Ultra App Kit and widget layouts.]

Are there plans to add alignment widgets to the toolkit?

Toolkits, such as NanoGUI, have layout widgets that control how the contained widgets are laid out and their centering.