NSWindow and Core Anim - flickering

FROM : John Clayton
DATE : Tue Nov 20 13:11:57 2007

Hello All,

I have an NSWindow instance that is entirely transparent (background 
color of the window has alpha with 0) which is sitting over another 
NSView that has its content rendered via Core Animation, lets call 
this the 'backdrop'.  The 'backdrop' is rendering arbitrarily complex 
CALayer hierarchies via CAOpenGLLayer.

On this 'overlay' NSWindow [that is sitting above the 'backdrop'], I 
have placed another NSView - lets call it the 'preview'.

When the 'preview' view is moved around, there is quite a lot of 
flickering going on, almost for every movement of the mouse (but not 
quite).  The flickering is restricted to the bounds of the 'preview' 
view.

If I change the code ever so slightly; and tell the 'overlay' that it 
wants a layer, like this:
   overlay.wantsLayer = YES;

then run the code again - the *entire* overlay window area flickers 
like mad.

I am more than happy to post snippets of code as required.  Does 
anyone have an idea of what this might be?


The window is init'd like this:

- (id)initWithContentRect:(NSRect)contentRect
               styleMask:(unsigned int)styleMask
                 backing:(NSBackingStoreType)bufferingType
                   defer:(BOOL)deferCreation
{
   NSWindow* me = [super initWithContentRect:contentRect
                           styleMask:NSBorderlessWindowMask
                             backing:NSBackingStoreBuffered
                               defer:YES];

   NSColor *blackTransparent =
       [NSColor colorWithDeviceRed:0.0 green:0.0 blue:0.0 alpha:0.0];
   [me setBackgroundColor:blackTransparent];
   
   [me setIsVisible:NO];
   [me setHasShadow:NO];
   [me setAcceptsMouseMovedEvents:YES];
   [me setExcludedFromWindowsMenu:YES];
   [me setIgnoresMouseEvents:NO];
   [me setMovableByWindowBackground:NO];
   [me setHidesOnDeactivate:NO];
   [me setOpaque:NO];
   
   // add a single view as the 'contentView', which is the same size as 
the window and stretches
   // appropriately to fit the content size
   NSRect viewFrame = contentRect;
   viewFrame.origin.x = 0;
   viewFrame.origin.y = 0;
   NSView* content = [[NSView alloc] initWithFrame:viewFrame];
   [content setAutoresizesSubviews:YES];
   [content setAutoresizingMask:NSViewWidthSizable | NSViewHeightSizable];
   [self setContentView:content];

     // if I enable this line below; flickering is seen across the 
entire NSWindow frame when the 'preview'
   // view is moved across it (this whole window is sitting on top of a 
layer-backed view)
   // content.wantsLayer = YES;
       
   return me;
}