Add logging to try to find cause of tabs out of order issue.

ThirdParty/PSMTabBarControl/source/PSMTabBarCell.m

@@ -245,6 +245,19 @@ static NSRect PSMConvertAccessibilityFrameToScreen(NSView *view, NSRect frame) {
     [super dealloc];
 }
+- (NSString *)description {
+    id identifier = nil;
+    if ([self.representedObject respondsToSelector:@selector(identifier)]) {
+        identifier = [self.representedObject identifier];
+    }
+    return [NSString stringWithFormat:@"<%@: %p representedObject=%@ identifier=%@ objectCount=%@>",
+            NSStringFromClass([self class]),
+            self,
+            self.representedObject,
+            identifier,
+            @(self.count)];
+}
+
 // we don't want this to be the first responder in the chain
 - (BOOL)acceptsFirstResponder {
   return NO;

ThirdParty/PSMTabBarControl/source/PSMTabBarControl.m

@@ -216,6 +216,31 @@ const NSInteger kPSMStartResizeAnimation = 0;
     }
 }
+- (void)sanityCheckFailedWithCallsite:(NSString *)callsite reason:(NSString *)reason {
+    ILog(@"Sanity check failed from %@ for reason %@. Cells=%@. tabView.tabViewItems=%@ stack:\n%@",
+         callsite,
+         reason,
+         self.cells,
+         self.tabView.tabViewItems,
+         [NSThread callStackSymbols]);
+}
+
+- (void)sanityCheck:(NSString *)callsite {
+    if ([[PSMTabDragAssistant sharedDragAssistant] isDragging]) {
+        return;
+    }
+    if (self.tabView.tabViewItems.count != self.cells.count) {
+        [self sanityCheckFailedWithCallsite:callsite reason:@"count mismatch"];
+    } else {
+        for (NSInteger i = 0; i < self.cells.count; i++) {
+            NSTabViewItem *tabViewItem = self.tabView.tabViewItems[i];
+            PSMTabBarCell *cell = self.cells[i];
+            if (cell.representedObject != tabViewItem) {
+                [self sanityCheckFailedWithCallsite:callsite reason:@"cells[i].representedObject != tabView.tabViewItems[i].representedObject"];
+            }
+        }
+    }
+}
 #pragma mark -
 #pragma mark Accessors

ThirdParty/PSMTabBarControl/source/PSMTabDragAssistant.m

@@ -437,6 +437,8 @@
                                  didDropTabViewItem:[[self draggedCell] representedObject]
                                            inTabBar:[self destinationTabBar]];
         }
+        [[self destinationTabBar] sanityCheck:@"destination performDragOperation"];
+        [[self sourceTabBar] sanityCheck:@"source performDragOperation"];
     }
     [[NSNotificationCenter defaultCenter] postNotificationName:PSMTabDragDidEndNotification object:nil];
@@ -490,16 +492,19 @@
                          didDropTabViewItem:[[self draggedCell] representedObject]
                                    inTabBar:control];
                 }
+                [control sanityCheck:@"add dragged tab to new window"];
             } else {
                 NSLog(@"Delegate returned no control to add to.");
                 [[[self sourceTabBar] cells] insertObject:[self draggedCell] atIndex:[self draggedCellIndex]];
                 [[[self sourceTabBar] window] setAlphaValue:1];  // Make the window visible again.
                 [[[self sourceTabBar] window] orderFront:nil];
+                [[self sourceTabBar] sanityCheck:@"delegate returned no control to add to"];
             }
         } else {
             // put cell back
             [[[self sourceTabBar] cells] insertObject:[self draggedCell] atIndex:[self draggedCellIndex]];
+            [[self sourceTabBar] sanityCheck:@"put cell back"];
         }
         [[NSNotificationCenter defaultCenter] postNotificationName:PSMTabDragDidEndNotification object:nil];
@@ -542,6 +547,8 @@
     [_sineCurveWidths removeAllObjects];
     [self setTargetCell:nil];
     self.temporarilyHiddenWindow = nil;
+    [[self sourceTabBar] sanityCheck:@"finishDrag source"];
+    [[self destinationTabBar] sanityCheck:@"finishDrag destination"];
 }
 - (void)draggingBeganAt:(NSPoint)aPoint {
@@ -620,7 +627,7 @@
 - (void)calculateDragAnimationForTabBar:(PSMTabBarControl *)control {
     BOOL removeFlag = YES;
-    NSMutableArray *cells = [control cells];
+    NSArray *cells = [control cells];
     int i, cellCount = [cells count];
     float position = [control orientation] == PSMTabBarHorizontalOrientation ? [[control style] leftMarginForTabBarControl] : [[control style] topMarginForTabBarControl];
@@ -721,6 +728,7 @@
     [[control cells] replaceObjectAtIndex:cellIndex withObject:pc];
     [[control cells] removeObjectAtIndex:(cellIndex + 1)];
     [[control cells] removeObjectAtIndex:(cellIndex - 1)];
+    ILog(@"distributePlaceholdersInTabBar:withDraggedCell:%@", cell);
     return;
 }
@@ -750,6 +758,7 @@
     } else {
         [[control cells] addObject:pc];
     }
+    ILog(@"distributePlaceholdersInTabBar draggedCell=%@", draggedCell);
 }
 - (void)removeAllPlaceholdersFromTabBar:(PSMTabBarControl *)control {

ThirdParty/PSMTabBarControl/source/PSMTabStyle.h

@@ -59,5 +59,6 @@ Protocol to be observed by all style delegate objects.  These objects handle the
 @interface PSMTabBarControl (StyleAccessors)
 - (NSMutableArray *)cells;
+- (void)sanityCheck:(NSString *)callsite;
 @end

sources/PTYTab.m

@@ -383,6 +383,14 @@ static void SetAgainstGrainDim(BOOL isVertical, NSSize *dest, CGFloat value) {
     [super dealloc];
 }
+- (NSString *)description {
+    return [NSString stringWithFormat:@"<%@: %p label=%@ objectCount=%@>",
+            NSStringFromClass([self class]),
+            self,
+            tabViewItem_.label,
+            @(objectCount_)];
+}
+
 #pragma mark - NSCopying
 - (id)copyWithZone:(NSZone *)zone {