Step 176: smooth UI transitions

editor/src/state/UIAnimationState.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#include <string>
+#include <algorithm>
+
+struct UIAnimationState {
+    std::string activeTabPath;
+    double tabSwitchTime = 0.0;
+
+    void recordTabSwitch(const std::string& path, double nowSeconds) {
+        if (path == activeTabPath) return;
+        activeTabPath = path;
+        tabSwitchTime = nowSeconds;
+    }
+
+    float tabFadeAlpha(const std::string& path,
+                       double nowSeconds,
+                       bool reduceMotion,
+                       float durationSeconds = 0.15f) const {
+        if (reduceMotion) return 1.0f;
+        if (path.empty() || path != activeTabPath) return 1.0f;
+        double elapsed = nowSeconds - tabSwitchTime;
+        float t = (durationSeconds <= 0.0f) ? 1.0f : (float)(elapsed / durationSeconds);
+        return std::max(0.0f, std::min(1.0f, t));
+    }
+};