容器首帧无尺寸时按真实比例重算取景

- 取景原先只在加载完成时按 measureViewport() 的宽高比算一次，隐藏面板里挂载时只能拿到方形兜底值
- 抽出 computeFraming(aspect)，ResizeObserver 首次量到非零尺寸后按真实比例重算并复位一次，之后不再覆盖用户的缩放

packages/model3d-viewer/src/scene.ts

@@ -272,25 +272,34 @@ export async function createModel3dViewerScene(
   abortIfNeeded();
 
   const bounds = new THREE.Box3().setFromObject(model3d);
-  const size = measureViewport();
-  const fitBox = {
-    min: { x: bounds.min.x, y: bounds.min.y, z: bounds.min.z },
-    max: { x: bounds.max.x, y: bounds.max.y, z: bounds.max.z },
-  };
-  const fitCenter = computeModel3dViewerBoxCenter(fitBox);
-  const fitSolver = createModel3dViewerFitSolver({
-    center: fitCenter,
-    aspect: size.aspect,
-    fovDegrees: MODEL3D_VIEWER_CAMERA_FOV_DEGREES,
-    fallbackRadius: computeModel3dViewerBoxRadius(fitBox),
-  });
-  feedModel3dViewerFitSolver(model3d, (x, y, z) => fitSolver.add(x, y, z));
-  const framing = computeModel3dViewerFraming({
-    box: fitBox,
-    aspect: size.aspect,
-    fovDegrees: MODEL3D_VIEWER_CAMERA_FOV_DEGREES,
-    distance: fitSolver.resolve(),
-  });
+  const viewport = measureViewport();
+
+  /**
+   * 按给定宽高比重算取景。容器在隐藏面板里挂载时首帧量不到尺寸，只能先按方形兜底，
+   * 等容器真正有尺寸后必须用真实比例重算，否则模型会按方形取景、被裁掉或显得过小。
+   */
+  function computeFraming(aspect: number) {
+    const fitBox = {
+      min: { x: bounds.min.x, y: bounds.min.y, z: bounds.min.z },
+      max: { x: bounds.max.x, y: bounds.max.y, z: bounds.max.z },
+    };
+    const fitSolver = createModel3dViewerFitSolver({
+      center: computeModel3dViewerBoxCenter(fitBox),
+      aspect,
+      fovDegrees: MODEL3D_VIEWER_CAMERA_FOV_DEGREES,
+      fallbackRadius: computeModel3dViewerBoxRadius(fitBox),
+    });
+    feedModel3dViewerFitSolver(model3d, (x, y, z) => fitSolver.add(x, y, z));
+    return computeModel3dViewerFraming({
+      box: fitBox,
+      aspect,
+      fovDegrees: MODEL3D_VIEWER_CAMERA_FOV_DEGREES,
+      distance: fitSolver.resolve(),
+    });
+  }
+
+  let framing = computeFraming(viewport.aspect);
+  let hasFramedViewport = viewport.width > 0 && viewport.height > 0;
 
   function measureViewport(): {
     width: number;
@@ -342,6 +351,17 @@ export async function createModel3dViewerScene(
   if (typeof ResizeObserver !== 'undefined') {
     resizeObserver = new ResizeObserver(() => {
       applyViewport();
+      if (disposed || hasFramedViewport) {
+        return;
+      }
+      // 首次拿到非零尺寸：按真实比例重算取景并复位，用户此后的缩放不再被覆盖。
+      const current = measureViewport();
+      if (current.width === 0 || current.height === 0) {
+        return;
+      }
+      hasFramedViewport = true;
+      framing = computeFraming(current.aspect);
+      resetView();
     });
     resizeObserver.observe(container);
   }