BScrollBar: Use the minimum as the preferred size in the alternate axis.

This was the behavior before the commit yesterday that unified the
two methods of computing the preferred size. It seems some applications
expect this to be callable without a window lock, so this works when
the other method does not.

Fixes #16372.

src/kits/interface/ScrollBar.cpp

@@ -999,10 +999,10 @@ BScrollBar::GetPreferredSize(float* _width, float* _height)
 			*_width = B_V_SCROLL_BAR_WIDTH * scale;
 
 		if (_height)
-			*_height = Bounds().Height();
+			*_height = _MinSize().Height();
 	} else if (fOrientation == B_HORIZONTAL) {
 		if (_width)
-			*_width = Bounds().Width();
+			*_width = _MinSize().Width();
 
 		if (_height)
 			*_height = B_H_SCROLL_BAR_HEIGHT * scale;