egui ends a drag only on a button release or Escape. When the window
never receives the mouse-up, a drag such as a dock column's resize keeps
following the pointer with no button held.
On macOS, each frame's input now gets a release for every button egui
still holds when NSEvent.pressedMouseButtons reports no button down, at
the pointer's last position. Frames that carry a button event, and
control-channel input that holds a button between steps, are left as
they are.