Fix: Clear stale Skyboxify sky layers to avoid frametime spikes (#108)

src/main/java/dev/zelo/renderscale/compat/skyboxify/SkyboxifyCompat.java

@@ -0,0 +1,71 @@
+package dev.zelo.renderscale.compat.skyboxify;
+
+import dev.zelo.renderscale.Constants;
+import net.minecraft.client.Minecraft;
+
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+
+/**
+ * Skyboxify queues its sky layers every frame during extraction, but only clears the queue when the vanilla sky
+ * pass actually renders. When vanilla skips the sky (e.g. underwater), the queue keeps growing and is drawn all at
+ * once when the sky comes back, causing a large frametime spike that scales with the render resolution.
+ * Clearing the queue after the level has rendered is a no-op in normal frames.
+ * <p>
+ * Everything is reflective so Skyboxify stays an optional dependency.
+ */
+public final class SkyboxifyCompat {
+    private static boolean initialized;
+    private static Field skyRendererField;
+    private static Field featureRendererField;
+    private static Method clearMethod;
+
+    private SkyboxifyCompat() {
+    }
+
+    public static void clearStaleSkySubmits() {
+        if (!initialized) {
+            initialized = true;
+            init();
+        }
+        if (clearMethod == null) return;
+
+        try {
+            Object skyRenderer = skyRendererField.get(Minecraft.getInstance().levelRenderer);
+            if (skyRenderer == null) return;
+            Object featureRenderer = featureRendererField.get(skyRenderer);
+            if (featureRenderer != null) clearMethod.invoke(featureRenderer);
+        } catch (ReflectiveOperationException | RuntimeException e) {
+            clearMethod = null;
+            Constants.LOG.warn("Skyboxify compatibility disabled", e);
+        }
+    }
+
+    private static void init() {
+        try {
+            Class<?> featureRenderer = Class.forName("btw.lowercase.skyboxify.skybox.renderer.FeatureRenderer");
+            Class<?> skyRenderer = Class.forName("net.minecraft.client.renderer.SkyRenderer");
+
+            Field levelRendererSky = null;
+            for (Field field : Minecraft.getInstance().levelRenderer.getClass().getDeclaredFields()) {
+                if (field.getType() == skyRenderer) {
+                    levelRendererSky = field;
+                    break;
+                }
+            }
+            if (levelRendererSky == null) return;
+
+            Field renderer = skyRenderer.getDeclaredField("skyboxify$skyFeatureRenderer");
+            Method clear = featureRenderer.getMethod("clear");
+            levelRendererSky.setAccessible(true);
+            renderer.setAccessible(true);
+
+            skyRendererField = levelRendererSky;
+            featureRendererField = renderer;
+            clearMethod = clear;
+            Constants.LOG.info("Skyboxify detected: stale sky layers will be cleared each frame");
+        } catch (ReflectiveOperationException | LinkageError ignored) {
+            // Skyboxify is not installed or has a different layout
+        }
+    }
+}

src/main/java/dev/zelo/renderscale/mixin/MixinGameRenderer.java

@@ -86,5 +86,6 @@ private void handBack(CallbackInfo callbackInfo) {
         windowState.guiScale = renderScale$guiScale;
         //?}
         RenderScale.getInstance().setShouldScale(false);
+        dev.zelo.renderscale.compat.skyboxify.SkyboxifyCompat.clearStaleSkySubmits();
     }
 }