Fix frame resize flicker on macOS (bug#30699)

* src/nsterm.h (ns_enable_screen_updates): New function.
* src/nsterm.m (ns_enable_screen_updates):
(ns_disable_screen_updates): New functions.
(disable_screen_updates_count): Count of number of times we've called
NSDisableScreenUpdates.
(x_set_window_size): Disable screen updates when not in a live resize
loop.
* src/xdisp.c (redisplay_internal): Reenable screen updates when
redisplay doesn't complete due to a popup.
(unwind_redisplay): Reenable screen updates.

1 files changed, 46 insertions, 0 deletions

diff --git a/src/nsterm.m b/src/nsterm.m
index 75e0b837c67..f9107c43ce1 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -288,6 +288,9 @@ static BOOL gsaved = NO;
 static BOOL ns_fake_keydown = NO;
 #ifdef NS_IMPL_COCOA
 static BOOL ns_menu_bar_is_hidden = NO;
+
+/* The number of times NSDisableScreenUpdates has been called.  */
+static int disable_screen_updates_count = 0;
 #endif
 /*static int debug_lock = 0; */
 
@@ -727,6 +730,40 @@ ns_release_autorelease_pool (void *pool)
 }
 
 
+#ifdef NS_IMPL_COCOA
+/* Disabling screen updates can be used to make several actions appear
+   "atomic" to the end user.  It seems some actions can still update
+   the display, though.
+
+   When we re-enable screen updates the number of calls to
+   NSEnableScreenUpdates should match the number to
+   NSDisableScreenUpdates.
+
+   We use these functions to prevent the user seeing a blank frame
+   after it has been resized.  x_set_window_size disables updates and
+   when redisplay completes unwind_redisplay enables them again
+   (bug#30699).  */
+
+static void
+ns_disable_screen_updates (void)
+{
+  NSDisableScreenUpdates ();
+  disable_screen_updates_count++;
+}
+
+void
+ns_enable_screen_updates (void)
+/* Re-enable screen updates.  Called from unwind_redisplay.  */
+{
+  while (disable_screen_updates_count > 0)
+    {
+      NSEnableScreenUpdates ();
+      disable_screen_updates_count--;
+    }
+}
+#endif
+
+
 static BOOL
 ns_menu_bar_should_be_hidden (void)
 /* True, if the menu bar should be hidden.  */
@@ -1877,6 +1914,15 @@ x_set_window_size (struct frame *f,
 
   block_input ();
 
+#ifdef NS_IMPL_COCOA
+  /* To prevent showing the user a blank frame, stop updates being
+     flushed to the screen until after redisplay has completed.  This
+     breaks live resize (resizing with a mouse), so don't do it if
+     we're in a live resize loop.  */
+  if (![view inLiveResize])
+    ns_disable_screen_updates ();
+#endif
+
   if (pixelwise)
     {
       pixelwidth = FRAME_TEXT_TO_PIXEL_WIDTH (f, width);