image/loading: Adjust frame timing by accrued delay

This should fix animations systematically slowing down.

news.d/51.alpha/fixed-animation-dont-accumulate-delay

@@ -0,0 +1 @@
+Don't accumulate delays during animations. !596

src/decoder/tiling.rs

@@ -649,22 +649,24 @@ impl SharedFrameBuffer {
         tiling.unwrap()
     }
 
-    /// Return true if the next frame should be shown and removes the outdated
-    /// frame
-    pub fn frame_timeout(&self, elapsed: Duration) -> bool {
+    /// Remove current frame if frame delay elapsed
+    ///
+    /// Returns the amount of time the frame is shown 'too late'
+    pub fn frame_timeout(&self, elapsed: Duration) -> Option<Duration> {
         let images = &self.buffer.load().images;
 
         // Only move to next image if there is one buffered
         if images.len() > 1
-            && images
-                .front()
-                .is_some_and(|next_frame| elapsed >= next_frame.delay)
+            && let Some(next_frame) = images.front()
+            && elapsed >= next_frame.delay
         {
+            let delay = elapsed - next_frame.delay;
             self.next_frame();
-            return true;
+            let delay = delay.min(next_frame.delay);
+            return Some(delay);
         }
 
-        false
+        None
     }
 
     pub fn next_frame(&self) {

src/widgets/image.rs

@@ -253,8 +253,8 @@ mod imp {
 
         /// Ticks callback for animated image formats
         pub(super) tick_callback: RefCell<Option<gtk::TickCallbackId>>,
-        /// Frame block time for currently shown frame
-        pub(super) last_animated_frame: Cell<i64>,
+        /// Adjusted frame clock time for when the current frame was shown
+        pub(super) last_animated_frame: Cell<Option<i64>>,
 
         /// Gesture, zoom or rotate, used for the duration of the gesture
         pub(super) locked_gestured: Cell<Option<Gesture>>,

src/widgets/image/loading.rs

@@ -160,17 +160,26 @@ impl imp::LpImage {
 
         // Do not animate if not visible
         if !obj.is_mapped() {
+            self.last_animated_frame.set(None);
             return glib::ControlFlow::Continue;
         }
 
-        let elapsed = clock.frame_time() - self.last_animated_frame.get();
-        let duration = std::time::Duration::from_micros(elapsed as u64);
+        let duration = if let Some(last_animated_frame) = self.last_animated_frame.get() {
+            let elapsed = clock.frame_time() - last_animated_frame;
+            std::time::Duration::from_micros(elapsed as u64)
+        } else {
+            self.last_animated_frame.set(Some(clock.frame_time()));
+            std::time::Duration::ZERO
+        };
 
         // Check if it's time to show the next frame
-        if self.frame_buffer.frame_timeout(duration) {
+        if let Some(delay) = self.frame_buffer.frame_timeout(duration) {
             // Just draw since frame_timeout updated to new frame
             obj.queue_draw();
-            self.last_animated_frame.set(clock.frame_time());
+            // Adjust the time when the frame was shown by how much anwanted delay we
+            // accumulated
+            self.last_animated_frame
+                .set(Some(clock.frame_time() - delay.as_micros() as i64));
             if let Some(decoder) = self.decoder.borrow().as_ref() {
                 // Decode new frame and load it into buffer
                 decoder.fill_frame_buffer();