The portaled position:fixed add-to-conversation button stays pinned to its
viewport anchor while the sidebar keeps every tab mounted (tab switches only
set display:none; collapsing the panel translates it off-screen), so the
popup could outlive its editor surface. Extract the popup into
useSelectionPopup and close it on any mousedown outside the button, Escape,
document hidden, window blur, and - the tab-switch/collapse path - when an
IntersectionObserver reports the editor surface off-viewport.