ui: letting go of shift while dragging a buffer makes the buffer disappear

When I drag a buffer from one spot to the next using shift+drag, it will disappear and become unretrievable if I accidentally let go of shift mid-drag.

A silly screencast attached:

Reproduce the shift+drag interaction in the T text editor, especially releasing Shift before the drag ends. Trace the buffer-drag handling from that interaction and verify that the buffer remains retrievable after the modifier is released.

Written by the indexing model from the issue text.