Issue was that the left mouse press event was handled to activate the
dragging state, but not "consumed". So general view item handling would
attempt to force-activate it again, exiting the dragging state
immediately again.
Consume the event now, plus handle properly any similar unexpected cases
(with failing assert), where modal state would be exited because of an
unconsumed event.
Alternative solution to #124529, thanks to Pratik Borhade for finding
the root of the issue.
d6e9438e60