Fix bitmap re-use bug cross fading from thumb.

Author: sjudd

library/src/androidTest/java/com/bumptech/glide/request/ThumbnailRequestCoordinatorTest.java

@@ -295,6 +295,13 @@ public void testDoesNotClearThumbOnThumbRequestComplete() {
         verify(thumb, never()).clear();
     }
 
+    @Test
+    public void testDoesNotClearThumbOnFullComplete_whenThumbIsComplete() {
+        when(thumb.isComplete()).thenReturn(true);
+        coordinator.onRequestSuccess(full);
+        verify(thumb, never()).clear();
+    }
+
     @Test
     public void testDoesNotNotifyParentOnThumbRequestComplete() {
         coordinator = new ThumbnailRequestCoordinator(parent);

library/src/main/java/com/bumptech/glide/request/ThumbnailRequestCoordinator.java

@@ -66,7 +66,12 @@ public void onRequestSuccess(Request request) {
         if (coordinator != null) {
             coordinator.onRequestSuccess(this);
         }
-        thumb.clear();
+        // Clearing the thumb is not necessarily safe if the thumb is being displayed in the Target,
+        // as a layer in a cross fade for example. The only way we know the thumb is not being
+        // displayed and is therefore safe to clear is if the thumb request has not yet completed.
+        if (!thumb.isComplete()) {
+          thumb.clear();
+        }
     }
 
     private boolean parentIsAnyResourceSet() {