segmented_button: fix internal tab reorder end-to-end

Two independent bugs prevented tab drag-and-drop reorder from working
on cosmic-comp (and likely other compositors):

1. allow_reorder required DndAction::Move to be negotiated via
   OfferEvent::SelectedAction, which cosmic-comp does not always emit
   for self-drops (the SelectedAction event either never arrives or
   arrives with DndAction::empty()). Add a fallback: accept self-drops
   whenever state.dragging_tab is set. dragging_tab is only populated
   by start_tab_drag on this same widget, so this is safe; mime match
   and on_reorder presence are checked below.

2. reorder_event_for_drop preferred drop_hint.side over positional
   swap, producing counter-intuitive no-ops: dropping A (pos 0) on the
   left half of B (pos 1) resolved to "Before B" which, after removing
   A, lands at pos 0 again — the tab appeared not to move. Always use
   default_insert_position, which derives direction from dragged vs
   target positions (Konsole/Firefox/Chrome-style swap semantics).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/widget/segmented_button/widget.rs

@@ -406,11 +406,12 @@ where
         {
             return None;
         }
-        let position = state
+        // Always use positional swap (Konsole/Firefox/Chrome semantics):
-            .drop_hint
+        // dropping onto any part of a different tab swaps it with the dragged
-            .filter(|hint| hint.entity == target)
+        // tab. drop_hint.side-based Before/After is counter-intuitive: dropping
-            .map(|hint| InsertPosition::from(hint.side))
+        // A (pos 0) on the left half of B (pos 1) resolves to "Before B" which,
-            .unwrap_or_else(|| self.default_insert_position(dragged, target));
+        // after removing A, lands at pos 0 — so the tab appears not to move.
+        let position = self.default_insert_position(dragged, target);
         Some(ReorderEvent {
             dragged,
             target,
@@ -1201,7 +1202,14 @@ where
                         .dnd_state
                         .drag_offer
                         .as_ref()
-                        .is_some_and(|offer| offer.selected_action.contains(DndAction::Move));
+                        .is_some_and(|offer| offer.selected_action.contains(DndAction::Move))
+                        // Self-drop fallback: some compositors (cosmic-comp
+                        // observed) do not emit OfferEvent::SelectedAction for
+                        // internal drags, leaving selected_action empty.
+                        // dragging_tab is only set by start_tab_drag on this
+                        // same widget, so this covers the self-drop case
+                        // safely; mime and on_reorder are checked below.
+                        || state.dragging_tab.is_some();
                     let pending_reorder = if allow_reorder
                         && self.on_reorder.is_some()
                         && self.tab_drag.as_ref().is_some_and(|d| d.mime == *mime_type)