Some further investigation:
If you have 2 open windows, then there is no hang if you drag a tab from one window into the other window (with the "+" icon showing on drop). I believe this is why I incorrectly thought this hang was fixed - for anyone trying to reproduce this, ensure you are dragging the tab outside of all existing Orion windows, so you should get a new window on dropping the tab.
If you right click a tab, and pick "Move Tab > New Window", there is no hang, and it achieves the result you would expect, suggesting the issue is UI related rather than functionality-related.
The UI hang appears to pertain specifically to dropping a tab on a blank area of screen (i.e. not on an existing Orion window), and happens after the 'preview' of dropping the tab is rendered. Putting the tab back in the source window (without releasing the mouse) is also fine.
