sl0p: highlight the drop target while dragging a pane

Dragging a pane by its title was blind until release. The pane a drop would
land on is now highlighted with the theme's frame_highlight colour.

Reuses the existing hover highlight rather than adding a second highlighting
mechanism, but sets tab.mouse_hover_pane_id directly instead of going through
execute_update_hover, which is gated on the mouse_hover_effects option. During
a drag this is essential feedback rather than decoration, so it must not be
switchable off.

The pane being dragged is deliberately NOT highlighted when hovered: dropping
onto itself is a no-op, so showing no highlight is the honest signal that
releasing there changes nothing.

The highlight is cleared on drop, so a completed drag does not leave a pane
looking hovered.

Baselines held: zellij-server 1441/30.

1 files changed, 5 insertions, 0 deletions

diff --git a/README.md b/README.md
index 0190c975..3cc4f856 100644
--- a/README.md
+++ b/README.md
@@ -71,6 +71,11 @@ handles, so two-axis resize from the top still works. Plain top-edge resize is g
 which costs nothing in practice: every boundary is shared, so the boundary above a
 pane is the bottom edge of the pane above it and can be dragged from there.
 
+While dragging, the pane the drop would land on is highlighted with the theme's
+`frame_highlight` colour. Hovering back over the pane being dragged shows NO
+highlight, because dropping onto itself does nothing - the absence of a highlight
+is the signal that releasing there changes nothing.
+
 Dropping a pane on itself is a no-op, so a plain click still just focuses, and
 double-click-to-rename still works. Stacked panes are excluded — in a stack the
 title row is how you focus a member.