cursor: count a shape held again when its asked raster has a native cursor

A lookup that missed took the shape out of _nativeIds, but one that hit
did not put it back. A shape asked at a new raster, then at one it has a
native cursor for, still counted as waiting: a restore finishing after
the switch away kept its pixels in _awaitingNative instead of letting
them go. The set now says whether the raster asked last has a native
cursor, either way.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab

flutter/lib/models/model.dart

@@ -3619,8 +3619,13 @@ class CursorModel with ChangeNotifier {
 
   String nativeKey(CursorData cache, double scale) {
     final key = '${_keyScope}_${cache.updateGetKey(scale)}';
-    // A native cursor at another raster does not hold the pixels this one is made from.
-    if (!_cacheKeys.contains(key)) _nativeIds.remove(cache.id);
+    // A shape is held by a native cursor at the raster asked last; one at another raster does
+    // not hold the pixels this one is made from.
+    if (_cacheKeys.contains(key)) {
+      _nativeIds.add(cache.id);
+    } else {
+      _nativeIds.remove(cache.id);
+    }
     return key;
   }
 

flutter/test/cursor_replay_test.dart

@@ -267,6 +267,27 @@ void main() {
         isNotEmpty);
   });
 
+  test('a raster returned to counts again as held by its native cursor',
+      () async {
+    final cursor = ffi.cursorModel;
+    await _feed(ffi, 'A', size: 32);
+    buildCursorOfCache(cursor, 1.0, cursor.cache);
+    await _settle();
+    await _feed(ffi, 'B');
+    buildCursorOfCache(cursor, 1.0, cursor.cache);
+    await _settle();
+
+    _select(ffi, 'A');
+    ffi.cursor.holdFetches = true;
+    buildCursorOfCache(cursor, 0.5, cursor.cache); // no cursor at 0.5: fetched
+    buildCursorOfCache(cursor, 1.0, cursor.cache); // back at 1.0, which has one
+    _select(ffi, 'B');
+    ffi.cursor.answerFetches();
+    await _settle();
+    expect(cursor.cachedShape('A')!.hasPixels, isFalse,
+        reason: 'its native cursor at the raster asked last holds it');
+  });
+
   test('shapes waiting for a native cursor keep their pixels, within the limit',
       () async {
     const max = CursorModel.kRecentShapes;