diff --git a/frontend/canvas.html b/frontend/canvas.html
index 67fdcae..a3b6cca 100644
--- a/frontend/canvas.html
+++ b/frontend/canvas.html
@@ -2246,24 +2246,73 @@ Organization : OptiHK Limited
 
     // Displays generated layout SVG previews with zoom and pan controls.
     const LayoutSvgPreview = ({ page }) => {
-      const [layoutScale, setLayoutScale] = useState(100);
+      const [layoutScale, setLayoutScale] = useState(null);
+      const [previewViewport, setPreviewViewport] = useState({ width: 1, height: 1 });
+      const [svgSize, setSvgSize] = useState(null);
+      const previewCanvasRef = useRef(null);
       const previewBounds = useMemo(
         () => page.layoutBounds || calculateLayoutBounds(page),
         [page.layoutBounds, page.nodes, page.canvasSize]
       );
-      const normalizedScale = Math.min(800, Math.max(10, Number(layoutScale) || 100));
-      const stageWidth = Math.max(1, previewBounds.width) * normalizedScale / 100;
-      const stageHeight = Math.max(1, previewBounds.height) * normalizedScale / 100;
+      const minLayoutScale = 0.01;
+      const maxLayoutScale = 800;
+      const scalePrecision = 100;
+      const clampLayoutScale = (value, fallback = 100) => {
+        const numericValue = Number(value);
+        const scale = Number.isFinite(numericValue) && numericValue > 0 ? numericValue : fallback;
+        return Number(Math.min(maxLayoutScale, Math.max(minLayoutScale, scale)).toFixed(2));
+      };
+      const baseWidth = Math.max(1, svgSize?.width || previewBounds.width);
+      const baseHeight = Math.max(1, svgSize?.height || previewBounds.height);
+      const availableWidth = Math.max(1, previewViewport.width);
+      const availableHeight = Math.max(1, previewViewport.height);
+      const rawFitScalePercent = Math.min(availableWidth / baseWidth, availableHeight / baseHeight) * 100;
+      const fitScalePercent = clampLayoutScale(Math.floor(rawFitScalePercent * scalePrecision) / scalePrecision);
+      const normalizedScale = clampLayoutScale(layoutScale ?? fitScalePercent, fitScalePercent);
+      const stageWidth = baseWidth * normalizedScale / 100;
+      const stageHeight = baseHeight * normalizedScale / 100;
+
+      useEffect(() => {
+        const previewCanvas = previewCanvasRef.current;
+        if (!previewCanvas) return undefined;
+
+        const measurePreviewViewport = () => {
+          const styles = window.getComputedStyle(previewCanvas);
+          const paddingX = (parseFloat(styles.paddingLeft) || 0) + (parseFloat(styles.paddingRight) || 0);
+          const paddingY = (parseFloat(styles.paddingTop) || 0) + (parseFloat(styles.paddingBottom) || 0);
+          setPreviewViewport({
+            width: Math.max(1, previewCanvas.clientWidth - paddingX),
+            height: Math.max(1, previewCanvas.clientHeight - paddingY)
+          });
+        };
+
+        measurePreviewViewport();
+        if (typeof ResizeObserver === 'undefined') {
+          window.addEventListener('resize', measurePreviewViewport);
+          return () => window.removeEventListener('resize', measurePreviewViewport);
+        }
+
+        const observer = new ResizeObserver(measurePreviewViewport);
+        observer.observe(previewCanvas);
+        return () => observer.disconnect();
+      }, []);
 
       const updateScale = (value) => {
-        setLayoutScale(Math.min(800, Math.max(10, Number(value) || 100)));
+        setLayoutScale(clampLayoutScale(value, fitScalePercent));
       };
 
       const handleWheel = (event) => {
         event.preventDefault();
         const direction = event.deltaY > 0 ? -1 : 1;
         const step = event.shiftKey ? 5 : 15;
-        setLayoutScale(current => Math.min(800, Math.max(10, (Number(current) || 100) + direction * step)));
+        setLayoutScale(current => clampLayoutScale((current ?? fitScalePercent) + direction * step, fitScalePercent));
+      };
+
+      const handleSvgLoad = (event) => {
+        const image = event.currentTarget;
+        if (image.naturalWidth > 0 && image.naturalHeight > 0) {
+          setSvgSize({ width: image.naturalWidth, height: image.naturalHeight });
+        }
       };
 
       return (
@@ -2273,9 +2322,9 @@ Organization : OptiHK Limited
               Scale
               <input
                 type="range"
-                min="10"
-                max="800"
-                step="5"
+                min={minLayoutScale}
+                max={maxLayoutScale}
+                step="0.01"
                 value={normalizedScale}
                 onChange={(event) => updateScale(event.target.value)}
                 aria-label="Layout SVG preview scale"
@@ -2284,9 +2333,9 @@ Organization : OptiHK Limited
             <label>
               <input
                 type="number"
-                min="10"
-                max="800"
-                step="5"
+                min={minLayoutScale}
+                max={maxLayoutScale}
+                step="0.01"
                 value={normalizedScale}
                 onChange={(event) => updateScale(event.target.value)}
                 aria-label="Layout SVG preview scale percent"
@@ -2294,7 +2343,7 @@ Organization : OptiHK Limited
               %
             </label>
           </div>
-          <div className="layout-preview-canvas" onWheel={handleWheel}>
+          <div className="layout-preview-canvas" ref={previewCanvasRef} onWheel={handleWheel}>
             <div className="layout-preview-scroll-area">
               <div
                 className="layout-preview-stage"
@@ -2304,6 +2353,7 @@ Organization : OptiHK Limited
                   className="layout-preview-image"
                   src={page.svgUrl}
                   alt={`${page.name} layout preview`}
+                  onLoad={handleSvgLoad}
                   style={{ objectFit: 'contain' }}
                 />
               </div>
diff --git a/tests/layout-ui-wiring.test.js b/tests/layout-ui-wiring.test.js
index 0a53352..89440e2 100644
--- a/tests/layout-ui-wiring.test.js
+++ b/tests/layout-ui-wiring.test.js
@@ -33,16 +33,6 @@ assert(
   'Build Layout should use the backend svg_url response'
 );
 assert(
-<<<<<<< HEAD
-  canvasHtml.includes('result.svg_ready && result.svg_url') &&
-    canvasHtml.includes('buildLayoutRequestRef') &&
-    canvasHtml.includes('buildLayoutBusyRef') &&
-    canvasHtml.includes("cache: 'no-store'"),
-  'Build Layout should wait for a ready, versioned SVG response and prevent stale duplicate preview updates'
-);
-assert(
-=======
->>>>>>> jingwen_main
   canvasHtml.includes('result.preview_error') &&
     canvasHtml.includes('Preview skipped: '),
   'Build Layout should log when the backend saves YAML but skips SVG preview because the router stack is unavailable'
@@ -60,8 +50,18 @@ assert(
   'layout SVG preview should expose an editable scale value'
 );
 assert(
-  canvasHtml.includes('objectFit: \'contain\''),
-  '100% layout preview scale should fit the full SVG within the screen'
+  canvasHtml.includes('objectFit: \'contain\'') &&
+    canvasHtml.includes('previewCanvasRef') &&
+    canvasHtml.includes('ResizeObserver') &&
+    canvasHtml.includes('svgSize') &&
+    canvasHtml.includes('naturalWidth') &&
+    canvasHtml.includes('naturalHeight') &&
+    canvasHtml.includes('const fitScalePercent = clampLayoutScale(Math.floor') &&
+    canvasHtml.includes('layoutScale ?? fitScalePercent') &&
+    canvasHtml.includes('const stageWidth = baseWidth * normalizedScale / 100') &&
+    !canvasHtml.includes('useState(100)') &&
+    !canvasHtml.includes('baseWidth * fitScale * normalizedScale / 100'),
+  'layout preview should default to the actual fitted SVG scale percent instead of redefining 100% as fit'
 );
 assert(
   canvasHtml.includes('className="build-gds-btn"'),
@@ -354,7 +354,7 @@ assert(
     canvasHtml.includes('layoutBounds') &&
     canvasHtml.includes('stageWidth') &&
     canvasHtml.includes('stageHeight'),
-  'layout preview should mouse-wheel zoom and size 100% from calculated box_size layout bounds'
+  'layout preview should mouse-wheel zoom from the actual calculated SVG scale'
 );
 assert(
   canvasHtml.includes('reactFlowInstance.fitBounds') &&