From c4b8f96d3c90ef6ba180d25fed5b697da4efb7bd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Benoi=CC=82t=20Rouleau?= <benoit.rouleau@icloud.com>
Date: Mon, 19 Oct 2026 11:31:04 -0400
Subject: [PATCH] metal: take a drawable only in frames that draw to the screen

`commit_frame` asked the view for its `currentDrawable` every frame and
presented it. `currentDrawable` waits in `-[CAMetalLayer nextDrawable]`
until the display lets go of one of the layer's drawables, so every
frame waited for the display, even one drawn only into render targets.

A frame now takes a drawable only when it begins a pass on the screen,
and `commit_frame` presents one only then. An app that draws its frames
off-screen and reads them back, such as a tool saving them to files,
can then draw them as fast as the GPU does. A frame with no pass at all
commits nothing, where it panicked on the missing command buffer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: multica-agent <github@multica.ai>
---
 src/graphics/metal.rs | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/src/graphics/metal.rs b/src/graphics/metal.rs
index e18ec9f75..5fa93db93 100644
--- a/src/graphics/metal.rs
+++ b/src/graphics/metal.rs
@@ -293,6 +293,9 @@ pub struct MetalContext {
     // / `end_render_pass`.
     current_pass_target: Option<Option<RenderPass>>,
     pending_end_encoder: bool,
+    /// Whether this frame drew to the screen, which takes one of the
+    /// view's drawables, so `commit_frame` presents it.
+    drew_to_screen: bool,
     view: ObjcId,
     device: ObjcId,
     current_frame_index: usize,
@@ -388,6 +391,7 @@ impl MetalContext {
                 current_pass_height: 0,
                 current_pass_target: None,
                 pending_end_encoder: false,
+                drew_to_screen: false,
                 view,
                 device,
                 buffers: vec![],
@@ -1312,6 +1316,7 @@ impl RenderingBackend for MetalContext {
 
             let (descriptor, width_px, height_px) = match pass {
                 None => {
+                    self.drew_to_screen = true;
                     // Read dimensions from the actual pass attachment
                     // texture, not `screen_size()`. MTKView's
                     // `drawableSize` and our cached screen size can
@@ -1454,12 +1459,18 @@ impl RenderingBackend for MetalContext {
         }
         unsafe {
             assert!(!self.command_queue.is_null());
-            let drawable: ObjcId = msg_send!(self.view, currentDrawable);
-            //msg_send_![drawable, retain];
-            msg_send_![self.command_buffer.unwrap(), presentDrawable: drawable];
-            msg_send_![self.command_buffer.unwrap(), commit];
-            msg_send_![self.command_buffer.unwrap(), waitUntilCompleted];
+            if let Some(command_buffer) = self.command_buffer {
+                // A frame drawn only off-screen leaves the drawables alone:
+                // `currentDrawable` waits until the display is done with one.
+                if self.drew_to_screen {
+                    let drawable: ObjcId = msg_send!(self.view, currentDrawable);
+                    msg_send_![command_buffer, presentDrawable: drawable];
+                }
+                msg_send_![command_buffer, commit];
+                msg_send_![command_buffer, waitUntilCompleted];
+            }
         }
+        self.drew_to_screen = false;
         for buffer in &mut self.buffers {
             buffer.next_value = 0;
         }