Rework somewhat how the folder menu window and main view sizes and positions ...

chrome/browser/cocoa/bookmark_bar_folder_controller.mm

Index: chrome/browser/cocoa/bookmark_bar_folder_controller.mm
===================================================================
--- chrome/browser/cocoa/bookmark_bar_folder_controller.mm	(revision 54167)
+++ chrome/browser/cocoa/bookmark_bar_folder_controller.mm	(working copy)
@@ -377,22 +377,22 @@
       [self adjustButtonWidths] + (2 * bookmarks::kBookmarkVerticalPadding) +
       bookmarks::kScrollViewContentWidthMargin;
   NSPoint newWindowTopLeft = [self windowTopLeftForWidth:windowWidth];
-  NSSize windowSize = [scrollView_ convertSize:NSMakeSize(windowWidth,
-                                                          windowHeight)
-                                        toView:nil];
-  newWindowTopLeft.y -= windowSize.height;
+  NSSize windowSize = NSMakeSize(windowWidth, windowHeight);
+  windowSize = [scrollView_ convertSize:windowSize toView:nil];
+  NSWindow* window = [self window];
+  // If the window is already visible then make sure its top remains stable.
+  BOOL windowAlreadyShowing = [window isVisible];
+  CGFloat deltaY = windowHeight - NSHeight([mainView_ frame]);
+  if (windowAlreadyShowing) {
+    NSRect oldFrame = [window frame];
+    newWindowTopLeft.y = oldFrame.origin.y + NSHeight(oldFrame);
+  }
   NSRect windowFrame = NSMakeRect(newWindowTopLeft.x,
-                                  newWindowTopLeft.y,
-                                  windowSize.width,
-                                  windowSize.height);
-
+      newWindowTopLeft.y - windowHeight, windowSize.width, windowHeight);
   // Make the scrolled content be the right size (full size).
-  NSRect mainViewFrame = NSMakeRect(0, 0,
-                                    windowWidth -
-                                    bookmarks::kScrollViewContentWidthMargin,
-                                    windowHeight);
+  NSRect mainViewFrame = NSMakeRect(0, 0, NSWidth(windowFrame) -
+      bookmarks::kScrollViewContentWidthMargin, NSHeight(windowFrame));
   [mainView_ setFrame:mainViewFrame];
-
   // Make sure the window fits on the screen.  If not, constrain.
   // We'll scroll to allow the user to see all the content.
   NSRect screenFrame = [[[self window] screen] frame];
@@ -404,17 +404,33 @@
   } else {
     scrollable_ = NO;
   }
-  NSWindow* window = [self window];
-  [window setFrame:windowFrame display:YES];
-  // If scrollable then offset the view and show the arrows.
+  [window setFrame:windowFrame display:NO];
   if (wasScrollable != scrollable_) {
-    NSSize windowLocalSize = [scrollView_ convertSize:windowFrame.size
-                                             fromView:nil];
-    [mainView_ scrollPoint:NSMakePoint(0, (NSHeight(mainViewFrame) -
-                                           windowLocalSize.height))];
+    // If scrollability changed then rework visibility of the scroll arrows
+    // and the scroll offset of the menu view.
+    NSSize windowLocalSize =
+        [scrollView_ convertSize:windowFrame.size fromView:nil];
+    CGFloat scrollPointY = NSHeight(mainViewFrame) - windowLocalSize.height +
+        bookmarks::kBookmarkVerticalPadding;
+    [mainView_ scrollPoint:NSMakePoint(0, scrollPointY)];
     [self showOrHideScrollArrows];
     [self addOrUpdateScrollTracking];
+  } else if (scrollable_ && windowAlreadyShowing) {
+    // If the window was already showing and is still scrollable then make
+    // sure the main view moves upward, not downward so that the content
+    // at the bottom of the menu, not the top, appears to move.
+    // The edge case is when the menu is scrolled all the way to top (hence
+    // the test of scrollDownArrowShown_) - don't scroll then.
+    NSView* superView = [mainView_ superview];
+    DCHECK([superView isKindOfClass:[NSClipView class]]);
+    NSClipView* clipView = static_cast<NSClipView*>(superView);
+    CGFloat scrollPointY = [clipView bounds].origin.y +
+        bookmarks::kBookmarkVerticalPadding;
+    if (scrollDownArrowShown_ || deltaY > 0.0)
+      scrollPointY += deltaY;
+    [mainView_ scrollPoint:NSMakePoint(0, scrollPointY)];
   }
+  [window display];
 }
 
 // Determine window size and position.
@@ -429,7 +445,8 @@
   int buttons = std::max(node->GetChildCount() - startingIndex, 1);
 
   // Prelim height of the window.  We'll trim later as needed.
-  int height = buttons * bookmarks::kBookmarkButtonHeight;
+  int height = buttons * bookmarks::kBookmarkButtonHeight +
+      bookmarks::kBookmarkVerticalPadding;
   // We'll need this soon...
   [self window];
 
@@ -1237,7 +1254,8 @@
   [self closeBookmarkFolder:self];
 
   // Prelim height of the window.  We'll trim later as needed.
-  int height = [buttons_ count] * bookmarks::kBookmarkButtonHeight;
+  int height = [buttons_ count] * bookmarks::kBookmarkButtonHeight +
+      bookmarks::kBookmarkVerticalPadding;
   [self adjustWindowForHeight:height];
 }
 
@@ -1370,7 +1388,8 @@
   }
 
   // Propose a height for the window.  We'll trim later as needed.
-  int height = buttonCount * bookmarks::kBookmarkButtonHeight;
+  int height = buttonCount * bookmarks::kBookmarkButtonHeight +
+      bookmarks::kBookmarkVerticalPadding;
   [self adjustWindowForHeight:height];
 }