Fix dock widgets not filling their complete available space

To repro:
 - stack 3 dock widgets vertically
 - close the top one
 - close the top one
 The remaining one should now fill the whole window but it wasn't.

Culprit was the code that makes sure that when a dock is closed that
the adjacent ones get to share their space

src/private/multisplitter/MultiSplitterLayout.cpp

@@ -1792,7 +1792,7 @@ void MultiSplitterLayout::updateAnchorFollowing(const AnchorGroup &groupBeingRem
             }
         }
 
-        if (doShift)
+        if (doShift && !anchorToShift->isFollowing())
             anchorToShift->setPosition(newPosition);
     }