To be clear, this is an old mockup (https://gitlab.gnome.org/Teams/Design/app-mockups/-/blob/master/mail/mail-adaptive.png this one) + some parts from current Geary + some improvisation, like the floating button. ^^
We just needed a way to test split view sizing with realistic content.
And here's how it resizes. The new part is that the sidebar size is dynamic.