Fix terminal text selection not auto-scrolling when dragging beyond bounds (#9448)

## Description

Fix terminal text selection no longer auto-scrolling downward when drag
exits the scroll container (block list).

**Root cause:** `LeftMouseDragged` events were subject to z-index
coverage checks in `DispatchedEvent::at_z_index()`. When the user
dragged downward past the block list, the cursor entered the area of
higher-z-index UI elements (input/footer), causing `is_covered()` to
return true and the drag events to be filtered out. This prevented the
block list element from receiving drag events, so its auto-scroll logic
(`mouse_dragged()`) never executed.

**Fix:** Bypass z-index filtering for `LeftMouseDragged` events in the
blocklist element specifically when selecting.

Fixes APP-3952

## Testing

Manual testing:
https://www.loom.com/share/0e81bc86229a4e0085b52253ca846346

## Agent Mode
- [x] Warp Agent Mode - This PR was created via Warp's AI Agent Mode

## Changelog Entries for Stable

CHANGELOG-BUG-FIX: Fixed terminal text selection not auto-scrolling when
dragging beyond bounds

---------

Co-authored-by: Oz <oz-agent@warp.dev>
Co-authored-by: oz-for-oss[bot] <277970191+oz-for-oss[bot]@users.noreply.github.com>

app/src/terminal/block_list_element.rs

@@ -4430,9 +4430,25 @@ impl Element for BlockListElement {
         app: &AppContext,
     ) -> bool {
         let z_index = self.child_max_z_index.expect("Z-index should exist.");
-        let Some(event_at_z_index) = event.at_z_index(z_index, ctx) else {
-            // Only proceed if there's a relevant event at this z-index.
-            return false;
+
+        // During an active text selection, bypass the z-index coverage check for
+        // drag events. The input/footer area below the block list is painted at a
+        // higher z-index, so `at_z_index` would filter out drags that cross into
+        // that region — breaking selection auto-scroll when dragging downward.
+        // This matches the pattern used by SelectableArea, Draggable, Resizable,
+        // and both Scrollable variants, which all use `raw_event()` for drags.
+        let event_at_z_index = if self.is_terminal_selecting
+            && matches!(
+                event.raw_event(),
+                Event::LeftMouseDragged { .. } | Event::LeftMouseUp { .. }
+            ) {
+            event.raw_event()
+        } else {
+            let Some(e) = event.at_z_index(z_index, ctx) else {
+                // Only proceed if there's a relevant event at this z-index.
+                return false;
+            };
+            e
         };
 
         let mut handled = false;