Enable window flushing after setting scroll position

The window flush was being enabled before the scroll position was set,
causing the preview to briefly display at position 0 (the flash).

Now:
1. Scale webview
2. Set scroll position
3. Refine with syncScrollers if needed
4. Enable window flushing (makes preview visible)

This ensures the preview is never visible at the wrong scroll position.

Related to #39

Author: claude

MacDown/Code/Document/MPDocument.m

@@ -231,10 +231,7 @@ static void (^MPGetPreviewLoadingCompletionHandler(MPDocument *doc))()
     return ^{
         WebView *webView = weakObj.preview;
         NSWindow *window = webView.window;
-        @synchronized(window) {
-            if (window.isFlushWindowDisabled)
-                [window enableFlushWindow];
-        }
+
         [weakObj scaleWebview];
 
         // Set initial scroll position to prevent flash to top
@@ -249,6 +246,12 @@ static void (^MPGetPreviewLoadingCompletionHandler(MPDocument *doc))()
             [weakObj updateHeaderLocations];
             [weakObj syncScrollers];
         }
+
+        // Enable window flushing AFTER scroll position is set
+        @synchronized(window) {
+            if (window.isFlushWindowDisabled)
+                [window enableFlushWindow];
+        }
     };
 }