Avoid screen artifacts with new OS X visible bell after scrolling

* src/nsterm.m (EmacsBell): Save NSView when displaying the
visible bell and set `needsDisplay' when removed.
(hide_bell): Trace.
(ns_copy_bits): Trace.

src/nsterm.m

@@ -1172,10 +1172,31 @@ static NSRect constrain_frame_rect(NSRect frameRect, bool isFullscreen)
    ========================================================================== */
 
 
+// This bell implementation shows the visual bell image asynchronously
+// from the rest of Emacs. This is done by adding a NSView to the
+// superview of the Emacs window and removing it using a timer.
+//
+// Unfortunately, some Emacs operations, like scrolling, is done using
+// low-level primitives that copy the content of the window, including
+// the bell image. To some extent, this is handled by removing the
+// image prior to scrolling and marking that the window is in need for
+// redisplay.
+//
+// To test this code, make sure that there is no artifacts of the bell
+// image in the following situations. Use a non-empty buffer (like the
+// tutorial) to ensure that a scroll is performed:
+//
+// * Single-window: C-g C-v
+//
+// * Side-by-windows: C-x 3 C-g C-v
+//
+// * Windows above each other: C-x 2 C-g C-v
+
 @interface EmacsBell : NSImageView
 {
   // Number of currently active bell:s.
   unsigned int nestCount;
+  NSView * mView;
   bool isAttached;
 }
 - (void)show:(NSView *)view;
@@ -1204,7 +1225,6 @@ - (id)init;
       [self.image unlockFocus];
 #else
       self.image = [NSImage imageNamed:NSImageNameCaution];
-      [self.image setScalesWhenResized:YES];
       [self.image setSize:NSMakeSize(self.image.size.width * 5,
                                      self.image.size.height * 5)];
 #endif
@@ -1229,6 +1249,7 @@ - (void)show:(NSView *)view
       [self setFrameSize:self.image.size];
 
       isAttached = true;
+      mView = view;
       [[[view window] contentView] addSubview:self
                                    positioned:NSWindowAbove
                                    relativeTo:nil];
@@ -1258,9 +1279,12 @@ - (void)hide
 
 -(void)remove
 {
+  NSTRACE ("[EmacsBell remove]");
   if (isAttached)
     {
+      NSTRACE_MSG ("removeFromSuperview");
       [self removeFromSuperview];
+      mView.needsDisplay = YES;
       isAttached = false;
     }
 }
@@ -1310,6 +1334,8 @@ static void hide_bell ()
      Ensure the bell is hidden.
    -------------------------------------------------------------------------- */
 {
+  NSTRACE ("hide_bell");
+
   if (bell_view != nil)
     {
       [bell_view remove];
@@ -2392,6 +2418,8 @@ static void hide_bell ()
 static void
 ns_copy_bits (struct frame *f, NSRect src, NSRect dest)
 {
+  NSTRACE ("ns_copy_bits");
+
   if (FRAME_NS_VIEW (f))
     {
       hide_bell();              // Ensure the bell image isn't scrolled.