Draw a rect in the uncovered region of the native scrollbar

When the requested region is larger than the native scrollbar the
uncovered regions need to be drawn with a background color to
avoid artifacts.

This situation can happen when the TabBar is inline with scrollbar
when the width of the TabBar is greater than the width of the
scrollbar. Inline TabBar support is added in a later commit.

Change-Id: I70cb2e5bfd8d79401f77bbe55abe596adc160194

vcl/source/control/scrbar.cxx

@@ -502,6 +502,20 @@ bool ScrollBar::ImplDrawNative( sal_uInt16 nDrawFlags )
         aCtrlRegion.Union( maPage1Rect );
         aCtrlRegion.Union( maPage2Rect );
         aCtrlRegion.Union( maThumbRect );
+
+        Rectangle aRequestedRegion(Point(0,0), GetOutputSizePixel());
+        // if the actual native control region is smaller then the region that
+        // we requested the control to draw in, then draw a background rectangle
+        // to avoid drawing artifacts in the uncovered region
+        if (aCtrlRegion.GetWidth() < aRequestedRegion.GetWidth() ||
+            aCtrlRegion.GetHeight() < aRequestedRegion.GetHeight())
+        {
+            Color aFaceColor = GetSettings().GetStyleSettings().GetFaceColor();
+            SetFillColor(aFaceColor);
+            SetLineColor(aFaceColor);
+            DrawRect(aRequestedRegion);
+        }
+
         bNativeOK = DrawNativeControl( CTRL_SCROLLBAR, (bHorz ? PART_DRAW_BACKGROUND_HORZ : PART_DRAW_BACKGROUND_VERT),
                         aCtrlRegion, nState, scrValue, OUString() );
     }
@@ -613,7 +627,7 @@ void ScrollBar::ImplDraw( sal_uInt16 nDrawFlags, OutputDevice* pOutDev )
 {
     DecorationView          aDecoView( pOutDev );
     Rectangle               aTempRect;
-    sal_uInt16                  nStyle;
+    sal_uInt16              nStyle;
     const StyleSettings&    rStyleSettings = pOutDev->GetSettings().GetStyleSettings();
     SymbolType              eSymbolType;
     bool                    bEnabled = IsEnabled();