Fix TabBar scrolling (#1178)

Task/Issue URL:
https://app.asana.com/0/1177771139624306/1204477216680148/f

**Description**:
- Fixes related to scrolling to Tab Bar selection 

**Steps to test this PR**:
1. Open many tabs to enter overflow mode
2. Reorder some tabs within the window
3. Validate tabs selection correctly adjusts tabs width
4. Drag very first tab to the far right of the tab bar - ensure TabBar
is scrolled to show the tab completely
5. Open new window
6. Select another tab in the first window - ensure its width is
correctly updated with animation

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Author: mallexxx

DuckDuckGo/TabBar/View/TabBarCollectionView.swift

@@ -56,7 +56,10 @@ final class TabBarCollectionView: NSCollectionView {
             os_log("TabBarCollectionView: More than 1 item or no item highlighted", type: .error)
             return
         }
+        scroll(to: indexPath)
+    }
 
+    func scroll(to indexPath: IndexPath) {
         let rect = frameForItem(at: indexPath.item)
         animator().performBatchUpdates({
             animator().scrollToVisible(rect)

DuckDuckGo/TabBar/View/TabBarViewController.swift

@@ -347,6 +347,7 @@ final class TabBarViewController: NSViewController {
             collectionView.animator().selectItems(at: [newSelectionIndexPath], scrollPosition: .centeredHorizontally)
         } else {
             collectionView.selectItems(at: [newSelectionIndexPath], scrollPosition: .centeredHorizontally)
+            collectionView.scrollToSelected()
         }
     }
 
@@ -659,7 +660,7 @@ extension TabBarViewController: TabCollectionViewModelDelegate {
         updateEmptyTabArea()
         hideTabPreview()
         if tabMode == .overflow {
-            scrollCollectionViewToEnd()
+            collectionView.scroll(to: IndexPath(item: index))
         }
     }
 
@@ -820,13 +821,7 @@ extension TabBarViewController: TabCollectionViewModelDelegate {
 extension TabBarViewController: NSCollectionViewDelegateFlowLayout {
 
     func collectionView(_ collectionView: NSCollectionView, layout collectionViewLayout: NSCollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> NSSize {
-        var isItemSelected = tabCollectionViewModel.selectionIndex == .unpinned(indexPath.item)
-        if TabDragAndDropManager.shared.sourceUnit?.tabCollectionViewModel === tabCollectionViewModel,
-           let draggingOverIndex = TabDragAndDropManager.shared.sourceUnit?.index {
-            // Drag&drop in progress - the empty space is equal to the selected tab width
-            isItemSelected = draggingOverIndex == indexPath.item
-        }
-
+        let isItemSelected = tabCollectionViewModel.selectionIndex == .unpinned(indexPath.item)
         return NSSize(width: self.currentTabWidth(selected: isItemSelected), height: TabBarViewItem.Height.standard.rawValue)
     }
 
@@ -975,9 +970,15 @@ extension TabBarViewController: NSCollectionViewDelegate {
         // dropping a tab, dropping of url handled in collectionView:acceptDrop:
         guard session.draggingPasteboard.types == [TabBarViewItemPasteboardWriter.utiInternalType] else { return }
 
+        defer {
+            TabDragAndDropManager.shared.clear()
+        }
         if case .private = operation {
             // Perform the drag and drop between multiple windows
             TabDragAndDropManager.shared.performDragAndDropIfNeeded()
+            DispatchQueue.main.async {
+                self.collectionView.scrollToSelected()
+            }
             return
         }
         // dropping not on a tab bar

DuckDuckGo/TabBar/View/TabDragAndDropManager.swift

@@ -50,17 +50,16 @@ final class TabDragAndDropManager {
 
     @discardableResult
     func performDragAndDropIfNeeded() -> Bool {
-        if let sourceUnit = sourceUnit, let destinationUnit = destinationUnit,
-           sourceUnit.tabCollectionViewModel !== destinationUnit.tabCollectionViewModel &&
-            sourceUnit.tabCollectionViewModel?.isBurner ==
-                    destinationUnit.tabCollectionViewModel?.isBurner {
+        if let sourceUnit = sourceUnit,
+           let destinationUnit = destinationUnit,
+           sourceUnit.tabCollectionViewModel !== destinationUnit.tabCollectionViewModel,
+           sourceUnit.tabCollectionViewModel?.isBurner == destinationUnit.tabCollectionViewModel?.isBurner {
 
             performDragAndDrop(from: sourceUnit, to: destinationUnit)
-            clear()
             return true
-        } else {
-            return false
         }
+
+        return false
     }
 
     private func performDragAndDrop(from sourceUnit: Unit, to destinationUnit: Unit) {
@@ -75,7 +74,7 @@ final class TabDragAndDropManager {
         sourceTabCollectionViewModel.moveTab(at: sourceUnit.index, to: destinationTabCollectionViewModel, at: newIndex)
     }
 
-    private func clear() {
+    func clear() {
         sourceUnit = nil
         destinationUnit = nil
     }