Fix an ordering problem that led to invalid edits in display map sync (#52930)

Co-authored-by: Cole Miller <cole@zed.dev>

Self-Review Checklist:

- [ ] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [ ] Performance impact has been considered and is acceptable

Fixes ZED-67X
Fixes ZED-50E

Release Notes:

- N/A

Co-authored-by: Cole Miller <cole@zed.dev>

crates/editor/src/display_map.rs

@@ -400,10 +400,13 @@ impl DisplayMap {
         diagnostics_max_severity: DiagnosticSeverity,
         cx: &mut Context<Self>,
     ) -> Self {
-        let buffer_subscription = buffer.update(cx, |buffer, _| buffer.subscribe());
-
         let tab_size = Self::tab_size(&buffer, cx);
+        // Important: obtain the snapshot BEFORE creating the subscription.
+        // snapshot() may call sync() which publishes edits. If we subscribe first,
+        // those edits would be captured but the InlayMap would already be at the
+        // post-edit state, causing a desync.
         let buffer_snapshot = buffer.read(cx).snapshot(cx);
+        let buffer_subscription = buffer.update(cx, |buffer, _| buffer.subscribe());
         let crease_map = CreaseMap::new(&buffer_snapshot);
         let (inlay_map, snapshot) = InlayMap::new(buffer_snapshot);
         let (fold_map, snapshot) = FoldMap::new(snapshot);
@@ -4079,4 +4082,64 @@ pub mod tests {
             chunks,
         );
     }
+
+    /// Regression test: Creating a DisplayMap when the MultiBuffer has pending
+    /// unsynced changes should not cause a desync between the subscription edits
+    /// and the InlayMap's buffer state.
+    ///
+    /// The bug occurred because:
+    /// 1. DisplayMap::new created a subscription first
+    /// 2. Then called snapshot() which synced and published edits
+    /// 3. InlayMap was created with the post-sync snapshot
+    /// 4. But the subscription captured the sync edits, leading to double-application
+    #[gpui::test]
+    fn test_display_map_subscription_ordering(cx: &mut gpui::App) {
+        init_test(cx, &|_| {});
+
+        // Create a buffer with some initial text
+        let buffer = cx.new(|cx| Buffer::local("initial", cx));
+        let multibuffer = cx.new(|cx| MultiBuffer::singleton(buffer.clone(), cx));
+
+        // Edit the buffer. This sets buffer_changed_since_sync = true.
+        // Importantly, do NOT call multibuffer.snapshot() yet.
+        buffer.update(cx, |buffer, cx| {
+            buffer.edit([(0..0, "prefix ")], None, cx);
+        });
+
+        // Create the DisplayMap. In the buggy code, this would:
+        // 1. Create subscription (empty)
+        // 2. Call snapshot() which syncs and publishes edits E1
+        // 3. Create InlayMap with post-E1 snapshot
+        // 4. Subscription now has E1, but InlayMap is already at post-E1 state
+        let map = cx.new(|cx| {
+            DisplayMap::new(
+                multibuffer.clone(),
+                font("Helvetica"),
+                px(14.0),
+                None,
+                1,
+                1,
+                FoldPlaceholder::test(),
+                DiagnosticSeverity::Warning,
+                cx,
+            )
+        });
+
+        // Verify initial state is correct
+        let snapshot = map.update(cx, |map, cx| map.snapshot(cx));
+        assert_eq!(snapshot.text(), "prefix initial");
+
+        // Make another edit
+        buffer.update(cx, |buffer, cx| {
+            buffer.edit([(7..7, "more ")], None, cx);
+        });
+
+        // This would crash in the buggy code because:
+        // - InlayMap expects edits from V1 to V2
+        // - But subscription has E1 ∘ E2 (from V0 to V2)
+        // - The calculation `buffer_edit.new.end + (cursor.end().0 - buffer_edit.old.end)`
+        //   would produce an offset exceeding the buffer length
+        let snapshot = map.update(cx, |map, cx| map.snapshot(cx));
+        assert_eq!(snapshot.text(), "prefix more initial");
+    }
 }