bug#42834: 28.0.50; macOS scroll and movement latency slows when display changes

[Alan Third]

On Wed, Aug 12, 2020 at 05:48:47PM -0400, Doug Davis wrote:
> > Do you see the same thing going the other way? (laptop screen -> monitor)
> 
> Ah yes, forgot to mention that- seeing it both ways.

Excellent!

> > Is your monitor a retina screen?
> 
> Yeah the MacBook is retina and the external display is 4k.

I think I know what's going on. If you don't use the exact same
settings for the drawing buffer as for the screen then some very slow
conversion processes kick in every time you flush the buffer to the
screen.

Odds are that the monitor and laptop screen have something slightly
different, probably a colorspace or something, so when switching from
one to the other the drawing buffer retains the settings for the old
screen.

Can you try the following change in nsterm.m?

@@ -8431,18 +8431,10 @@ - 
(void)windowDidChangeBackingProperties:(NSNotification *)notification
   if (! [self wantsUpdateLayer])
     return;
 
-  CGFloat old = [[[notification userInfo]
-                    objectForKey:@"NSBackingPropertyOldScaleFactorKey"]
-                  doubleValue];
-  CGFloat new = [[self window] backingScaleFactor];
-
-  if (old != new)
-    {
-      NSRect frame = [self frame];
-      [self createDrawingBuffer];
-      ns_clear_frame (emacsframe);
-      expose_frame (emacsframe, 0, 0, NSWidth (frame), NSHeight (frame));
-    }
+  NSRect frame = [self frame];
+  [self createDrawingBuffer];
+  ns_clear_frame (emacsframe);
+  expose_frame (emacsframe, 0, 0, NSWidth (frame), NSHeight (frame));
 }
 #endif /* NS_DRAW_TO_BUFFER */

-- 
Alan Third