Keep Metal enabled during normal tab drags; only tmux split-pane
drags fall back to legacy drawing. This avoids forcing every
session onto Core Text for the whole drag, which starved the main
thread and made the floating tab preview lag behind the cursor.

Use the live pointer for split-pane drop highlights during tab
drags, since AppKit's draggingLocation can be stale while
PSMTabDragAssistant is updating the floating drag window.

Add hysteresis around tab midpoints for reorder slots and around
split-half boundaries, reducing flicker.