Make horizontal scrollbar disappear when not needed.

Author: grokys

src/GitHub.UI/Controls/ScollingVerticalStackPanel.cs

@@ -160,7 +160,7 @@ protected override Size MeasureOverride(Size availableSize)
             ExtentWidth = maxWidth;
             ExtentHeight = height;
 
-            UpdateScrollInfo(new Size(maxWidth, height), new Size(ViewportWidth, ViewportHeight));
+            UpdateScrollInfo(new Size(maxWidth, height), availableSize);
 
             return new Size(
                 Math.Min(maxWidth, availableSize.Width),