While implementing a resizable layout in Discover using EuiResizableContainer, we ran into some conflicts with react-reverse-portal. We were able to implement a workaround, but it involved setting up our own event listeners to watch for when resizing starts and ends.

It would simplify our use case a lot, and would probably be generally useful, if EuiResizableContainer accepted callback props for onResizeStart and onResizeEnd that would allow consumers to easily listen for when resizing starts and ends.

For our use case it's only important to listen for resizing triggered by a mouse click, but it might be worth considering whether these callbacks would apply to keyboard based resizing as well.