There are two important concepts to understand in UI systems / Slate. Focus and Capture.
Focus is where keyboard input goes, and it never makes sense to have more than 1 focus per user of a UI.
Capture on the other hand is a concept for pointers|fingers|cursors. To allow you to trap the input and prevent other widgets from receiving that input until you’re done with it. It’s what allows buttons to know when the mouse has been released once it has been pressed in the bounds of the button and released outside of it.
In Slate, buttons default to being focusable. Focusable elements that lose focus while they have capture, by convention give up their capture as well. The usecase is something like, User 1 presses A to mash a button on the UI, but before releasing presses down to goto the next button, his pressed state/capture state needs to be broken.
If you need two buttons to both be interactive at the same time, set their focusable state to false in the details panel.
This catches people a lot on mobile, I’ve been thinking about making it so the IsFocusable option on buttons is more of a platform specific default.