Fixed: if CPArrayController setPreserveSelection was off and a selected item disappeared, an invalid selection state could occur. The error was caused by a slightly overeager early-out optimisation in __setSelectionIndexes.

AppKit/CPArrayController.j

@@ -363,9 +363,6 @@
     if (!indexes)
         indexes = [CPIndexSet indexSet];
 
-    if ([_selectionIndexes isEqualToIndexSet:indexes])
-        return NO;
-
     if (![indexes count])
     {
         if(_avoidsEmptySelection && [[self arrangedObjects] count])
@@ -381,6 +378,9 @@
             indexes = [CPIndexSet indexSetWithIndex:objectsCount-1];
     }
 
+    if ([_selectionIndexes isEqualToIndexSet:indexes])
+        return NO;
+
     _selectionIndexes = [indexes copy];
 
     // Push back the new selection to the model for selectionIndexes if we have one.

Tests/AppKit/CPArrayControllerTest.j

@@ -113,6 +113,25 @@
     [self assert:[CPIndexSet indexSet] equals:[arrayController selectionIndexes] message:@"no objects left, selection should disappear"];
 }
 
+- (void)testSelectionWhenObjectsDisappear
+{
+    // If the selected object disappeares during a rearrange, the selection
+    // should update appropriately, even if preserve selection is off.
+    var arrayController = [self arrayController];
+    [arrayController setPreservesSelection:NO];
+
+    // Use a copy to make sure our original remains pristine.
+    [arrayController setSelectionIndexes:[CPIndexSet indexSetWithIndexesInRange:CPMakeRange(0, 3)]];
+
+    var newContent = [[self contentArray] copy];
+    [newContent removeObjectAtIndex:2];
+
+    [arrayController setContent:newContent];
+
+    [self assert:[CPIndexSet indexSetWithIndexesInRange:CPMakeRange(0, 2)] equals:[arrayController selectionIndexes]
+         message:@"last object cannot be selected"];
+}
+
 - (void)testContentBinding
 {
     [[self arrayController] bind:@"contentArray" toObject:self withKeyPath:@"contentArray" options:0];