feat: add proper CADisplayLink handling for iOS15 where commandBuffer presentDrawable:drawable afterMinimumDuration doesn't seem to work well

VoidLink/Metal/MetalVideoRenderer.h

@@ -22,12 +22,21 @@
 @property (nonatomic) id<CAMetalDrawable> _Nullable nextDrawable;
 @property (atomic) BOOL isStopping;
 
+// iOS 15.0 and below frame timing properties
+@property (nonatomic) BOOL useLegacyFrameTiming;
+@property (nonatomic) CFTimeInterval lastDisplayLinkTime;
+@property (nonatomic) CFTimeInterval displayLinkFrametime;
+
 - (nonnull instancetype)initWithMetalDevice:(nonnull id<MTLDevice>)device drawablePixelFormat:(MTLPixelFormat)drawablePixelFormat framerate:(float)framerate;
 - (void)renderFrame:(nonnull Frame *)frame toLayer:(nonnull CAMetalLayer *)layer API_AVAILABLE(ios(13.0));
 - (void)waitToRenderTo:(nonnull CAMetalLayer *)layer API_AVAILABLE(ios(13.0));
 - (void)drawableResize:(CGSize)drawableSize;
 - (void)shutdown;
 
+// iOS 15.0 and below legacy timing support
+- (void)updateLegacyFrameTiming:(CFTimeInterval)displayLinkTimestamp;
+- (void)resetFrameTiming;
+
 + (NSString *_Nullable)currentColorSpace;
 
 @end

VoidLink/Metal/MetalVideoRenderer.m

@@ -157,6 +157,17 @@ CFStringRef __currentColorSpace;
         _lastPresented = 0.0f;
         _inFlightSemaphore = dispatch_semaphore_create(MaxFramesInFlight);
         _isStopping = NO;
+        
+        // Enable legacy frame timing for iOS 15.0 and below to fix timing issues
+        if (@available(iOS 16.0, *)) {
+            _useLegacyFrameTiming = NO;
+            Log(LOG_I, @"MetalVideoRenderer: Using iOS 16+ timing (Metal presentedTime)");
+        } else {
+            _useLegacyFrameTiming = YES;
+            _lastDisplayLinkTime = 0.0f;
+            _displayLinkFrametime = 1.0f / framerate * 1000.0f; // Expected frametime in ms
+            Log(LOG_I, @"MetalVideoRenderer: Using legacy timing for iOS 15.0 and below (CADisplayLink)");
+        }
 
         CFStringRef keys[1] = {kCVMetalTextureUsage};
         NSUInteger values[1] = {MTLTextureUsageShaderRead};
@@ -713,14 +724,21 @@ CFStringRef __currentColorSpace;
         [renderEncoder drawPrimitives:MTLPrimitiveTypeTriangleStrip vertexStart:0 vertexCount:4];
         [renderEncoder endEncoding];
 
-        __block MetalVideoRenderer *strongSelf = self;
 #if !TARGET_OS_SIMULATOR
+        __block MetalVideoRenderer *strongSelf = self;
         [drawable addPresentedHandler:^(id<MTLDrawable> d) {
-            if (strongSelf.lastPresented > 0.0f) {
-                CFTimeInterval frametime = d.presentedTime - strongSelf.lastPresented;
-                [[ImGuiPlots sharedInstance] observeFloat:PLOT_FRAMETIME value:(frametime * 1000.0)];
+            // Only use Metal's presentedTime on iOS 16+ due to timing bugs on iOS 15.0
+            if (!strongSelf.useLegacyFrameTiming) {
+                if (strongSelf.lastPresented > 0.0f) {
+                    CFTimeInterval frametime = d.presentedTime - strongSelf.lastPresented;
+                    // Sanity check: reject negative or impossibly large frametimes
+                    if (frametime > 0 && frametime < 0.1) { // Less than 100ms
+                        [[ImGuiPlots sharedInstance] observeFloat:PLOT_FRAMETIME value:(frametime * 1000.0)];
+                    }
+                }
+                strongSelf.lastPresented = d.presentedTime;
             }
-            strongSelf.lastPresented = d.presentedTime;
+            // For iOS 15.0 and below, frametime is calculated in updateLegacyFrameTiming
         }];
 #endif
 
@@ -777,6 +795,43 @@ CFStringRef __currentColorSpace;
     }
 }
 
+#pragma mark - iOS 15.0 and below legacy timing support
+
+- (void)updateLegacyFrameTiming:(CFTimeInterval)displayLinkTimestamp {
+    if (!_useLegacyFrameTiming) {
+        return; // Only for iOS 15.0 and below
+    }
+    
+    if (_lastDisplayLinkTime > 0.0f) {
+        CFTimeInterval actualFrametime = (displayLinkTimestamp - _lastDisplayLinkTime) * 1000.0; // Convert to ms
+        
+        // Sanity check: reject negative or impossibly large frametimes
+        if (actualFrametime > 0 && actualFrametime < 100.0) { // Less than 100ms
+            [[ImGuiPlots sharedInstance] observeFloat:PLOT_FRAMETIME value:actualFrametime];
+            _displayLinkFrametime = actualFrametime;
+        } else {
+            // Use expected frametime if actual is unreliable
+            Log(LOG_W, @"MetalVideoRenderer: Rejecting invalid frametime %.2f ms, using expected %.2f ms", actualFrametime, _displayLinkFrametime);
+            [[ImGuiPlots sharedInstance] observeFloat:PLOT_FRAMETIME value:_displayLinkFrametime];
+        }
+    }
+    
+    _lastDisplayLinkTime = displayLinkTimestamp;
+}
+
+- (void)resetFrameTiming {
+    if (_useLegacyFrameTiming) {
+        // Reset legacy timing state for iOS 15.0 and below
+        _lastDisplayLinkTime = 0.0f;
+        _displayLinkFrametime = 1.0f / _framerate * 1000.0f;
+        Log(LOG_I, @"MetalVideoRenderer: Reset legacy frame timing for iOS 15.0 and below");
+    } else {
+        // Reset Metal timing state for iOS 16+
+        _lastPresented = 0.0f;
+        Log(LOG_I, @"MetalVideoRenderer: Reset Metal frame timing for iOS 16+");
+    }
+}
+
 /// Responds to the drawable's size or orientation changes.
 - (void)drawableResize:(CGSize)drawableSize {
     [self resize:drawableSize];

VoidLink/Metal/MetalViewController.h

@@ -22,4 +22,7 @@
                             enableHdr:(BOOL)enableHdr
                        metricsHandler:(MetricsHandler _Nonnull)metricsHandler;
 
+// Reset frame timing for iOS 15.0 and below compatibility
+- (void)resetFrameTiming;
+
 @end

VoidLink/Metal/MetalViewController.m

@@ -92,19 +92,35 @@
     view.metalLayer.pixelFormat = renderer.colorPixelFormat;
     view.metalLayer.maximumDrawableCount = 3;
 
-    // We need a no-op displaylink timer or iOS can decide to run at 60fps
-    // The overhead from this should be minimal.
+    // We need a displaylink timer for iOS 15.0 and below to provide accurate frame timing,
+    // or just as a no-op for iOS 16+ to prevent iOS from dropping to 60fps
     _displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(displayLinkHandler:)];
-    if (@available(iOS 15.0, tvOS 15.0, *)) {
+    if (@available(iOS 16.0, tvOS 16.0, *)) {
+        // iOS 16+: Use CAFrameRateRange, displaylink is just a no-op
         _displayLink.preferredFrameRateRange = CAFrameRateRangeMake(_framerate, _framerate, _framerate);
+        Log(LOG_I, @"MetalViewController: DisplayLink using iOS 16+ CAFrameRateRange");
+    } else if (@available(iOS 15.0, tvOS 15.0, *)) {
+        // iOS 15.0: Use legacy preferredFramesPerSecond to avoid CAFrameRateRange bugs
+        _displayLink.preferredFramesPerSecond = _framerate;
+        Log(LOG_I, @"MetalViewController: DisplayLink using iOS 15.0 legacy preferredFramesPerSecond");
     } else {
+        // iOS 14 and below
         _displayLink.preferredFramesPerSecond = _framerate;
     }
     [_displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
 }
 
 - (void)displayLinkHandler:(CADisplayLink *)link {
-    // Rendering does not use DisplayLink, this exists to fool iOS into keeping us running at the desired framerate
+    // For iOS 16+: This is just a no-op to fool iOS into keeping us running at the desired framerate
+    // For iOS 15.0 and below: Update renderer with accurate frame timing to fix negative framerates
+    if (@available(iOS 16.0, *)) {
+        // No-op for iOS 16+, Metal's presentedTime is reliable
+    } else {
+        // iOS 15.0 and below: Use DisplayLink timing since Metal's presentedTime is unreliable
+        if (_renderer) {
+            [_renderer updateLegacyFrameTiming:link.timestamp];
+        }
+    }
 }
 
 - (void)waitToRenderTo:(nonnull CAMetalLayer *)layer {
@@ -154,4 +170,32 @@
 }
 #endif
 
+- (void)resetFrameTiming {
+    // Reset the renderer's frame timing state for iOS 15.0 and below compatibility
+    if (_renderer) {
+        [_renderer resetFrameTiming];
+        Log(LOG_I, @"MetalViewController: Reset frame timing called");
+    }
+    
+    // Reset DisplayLink timing for iOS 15.0 and below
+    if (@available(iOS 16.0, *)) {
+        // No additional DisplayLink reset needed for iOS 16+
+    } else {
+        // For iOS 15.0 and below, pause and resume DisplayLink to re-engage timing
+        if (_displayLink) {
+            _displayLink.paused = YES;
+            
+            // Re-apply frame rate (avoid CAFrameRateRange on iOS 15.0)
+            if (@available(iOS 15.0, *)) {
+                _displayLink.preferredFramesPerSecond = _framerate;
+            } else {
+                _displayLink.preferredFramesPerSecond = _framerate;
+            }
+            
+            _displayLink.paused = NO;
+            Log(LOG_I, @"MetalViewController: Reset DisplayLink timing for iOS 15.0 and below");
+        }
+    }
+}
+
 @end

VoidLink/ViewControllers/StreamFrameViewController.m

@@ -1043,6 +1043,12 @@
     }
     
     [self->_streamMan.videoRenderer resetFramePacing];
+    
+    // Also reset Metal renderer timing for iOS 15.0 and below compatibility
+    if (self.metalViewController) {
+        [self.metalViewController resetFrameTiming];
+    }
+    
     _isRestoringFromPiP = NO;
 }