Avoid the explicit CATransaction

CHANGELOG.md

@@ -17,6 +17,7 @@
 - **Breaking:** Removed `DamageOutOfRange` error case. If the damage value is greater than the backend supports, it is instead clamped to an appropriate value.
 - **Breaking:** Removed `SurfaceExtWeb` and the associated `NoDisplayHandle` and `NoWindowHandle` helpers. Use `RawWindowHandle::WebCanvas` or `RawWindowHandle::WebOffscreenCanvas` instead.
 - Fixed `present_with_damage` with bounds out of range on Windows, Web and X11.
+- Reduced flickering when presenting while resizing on macOS.
 
 # 0.4.8
 

src/backends/cg.rs

@@ -418,14 +418,30 @@ impl BufferInterface for BufferImpl<'_> {
         }
         .unwrap();
 
-        // Wrap layer modifications in a transaction. Unclear if we should keep doing this, see
-        // <https://github.com/rust-windowing/softbuffer/pull/275> for discussion about this.
-        CATransaction::begin();
+        // Wrap things in a transaction if the event loop isn't currently running.
+        //
+        // The event loop implicitly commits pending layer updates at the end of the current run
+        // loop iteration, as documented in:
+        // <https://developer.apple.com/documentation/quartzcore/catransaction?language=objc>
+        //
+        //
+        //
+        // This is a bit overly conservative; the full check on macOS could. But it doesn't matter _that_ much, if users decide to render in weird ways, we'll still support it when emitting a transaction, it'll just be slightly less efficient.
+        let use_transaction = objc2_core_foundation::CFRunLoop::current()
+            .unwrap()
+            .current_mode()
+            .is_none();
+
+        if use_transaction {
+            CATransaction::begin();
+        }
 
         // SAFETY: The contents is `CGImage`, which is a valid class for `contents`.
         unsafe { layer.setContents(Some(image.as_ref())) };
 
-        CATransaction::commit();
+        if use_transaction {
+            CATransaction::commit();
+        }
 
         Ok(())
     }

src/lib.rs

@@ -352,6 +352,21 @@ impl Buffer<'_> {
     ///
     /// If the caller wishes to synchronize other surface/window changes, such requests must be sent to the
     /// Wayland compositor before calling this function.
+    ///
+    /// ## macOS / iOS
+    ///
+    /// On macOS/iOS/etc., this sets the [contents] of the underlying [`CALayer`], but doesn't yet
+    /// actually commit those contents to the compositor; that is instead done automatically by
+    /// QuartzCore at the end of the current iteration of the runloop. This synchronizes the
+    /// contents with the rest of the window, which is important to avoid flickering when resizing.
+    ///
+    /// If you need to send the contents to the compositor immediately (might be useful when
+    /// rendering from a separate thread or when using Softbuffer without the standard AppKit/UIKit
+    /// runloop), you'll want to wrap this function in a [`CATransaction`].
+    ///
+    /// [contents]: https://developer.apple.com/documentation/quartzcore/calayer/contents?language=objc
+    /// [`CALayer`]: https://developer.apple.com/documentation/quartzcore/calayer?language=objc
+    /// [`CATransaction`]: https://developer.apple.com/documentation/quartzcore/catransaction?language=objc
     #[inline]
     pub fn present(self) -> Result<(), SoftBufferError> {
         // Damage the entire buffer.