From 68bd6f3ea9c05637501139c46f1f4304482db95f Mon Sep 17 00:00:00 2001
From: Alan Third <alan@idiocy.org>
Date: Sun, 31 Jan 2021 20:19:53 +0000
Subject: [PATCH] Fix flicker when resizing NS frame programmatically
 (bug#46155)

; Incidentally fixes bug#21326.

* src/nsterm.m ([EmacsView viewWillDraw]): New function.
([EmacsView viewDidResize:]): We now have to mark the frame for
display on resize.
([EmacsView initFrameFromEmacs:]): Retain frame contents on resize.
([EmacsView updateLayer]): Don't update the layer if the frame is
still garbaged.
---
 src/nsterm.m | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/src/nsterm.m b/src/nsterm.m
index ca240eb55f1..b0cf5952fd5 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -7350,6 +7350,8 @@ not_in_argv (NSString *arg)
 
       [surface release];
       surface = nil;
+
+      [self setNeedsDisplay:YES];
     }
 #endif
 
@@ -7521,6 +7523,16 @@ not_in_argv (NSString *arg)
   [self initWithFrame: r];
   [self setAutoresizingMask: NSViewWidthSizable | NSViewHeightSizable];
 
+#ifdef NS_DRAW_TO_BUFFER
+  /* These settings mean AppKit will retain the contents of the frame
+     on resize.  Unfortunately it also means the frame will not be
+     automatically marked for display, but we can do that ourselves in
+     viewDidResize.  */
+  [self setLayerContentsRedrawPolicy:
+          NSViewLayerContentsRedrawOnSetNeedsDisplay];
+  [self setLayerContentsPlacement:NSViewLayerContentsPlacementTopLeft];
+#endif
+
   FRAME_NS_VIEW (f) = self;
   emacsframe = f;
 #ifdef NS_IMPL_COCOA
@@ -8463,6 +8475,34 @@ not_in_argv (NSString *arg)
 }
 
 
+#ifdef NS_IMPL_COCOA
+/* If the frame has been garbaged but the toolkit wants to draw, for
+   example when resizing the frame, we end up with a blank screen.
+   Sometimes this results in an unpleasant flicker, so try to
+   redisplay before drawing.  */
+- (void)viewWillDraw
+{
+  if (FRAME_GARBAGED_P (emacsframe)
+      && !redisplaying_p)
+    {
+      /* If there is IO going on when redisplay is run here Emacs
+         crashes.  I think it's because this code will always be run
+         within the run loop and for whatever reason processing input
+         is dangerous.  This technique was stolen wholesale from
+         nsmenu.m and seems to work.  */
+      bool owfi = waiting_for_input;
+      waiting_for_input = 0;
+      block_input ();
+
+      redisplay ();
+
+      unblock_input ();
+      waiting_for_input = owfi;
+    }
+}
+#endif
+
+
 #ifdef NS_DRAW_TO_BUFFER
 - (BOOL)wantsUpdateLayer
 {
@@ -8480,6 +8520,13 @@ not_in_argv (NSString *arg)
 {
   NSTRACE ("[EmacsView updateLayer]");
 
+  /* We run redisplay on frames that are garbaged, but marked for
+     display, before updateLayer is called so if the frame is still
+     garbaged that means the last redisplay must have refused to
+     update the frame.  */
+  if (FRAME_GARBAGED_P (emacsframe))
+    return;
+
   /* This can fail to update the screen if the same surface is
      provided twice in a row, even if its contents have changed.
      There's a private method, -[CALayer setContentsChanged], that we
-- 
2.39.2