Do not fall back after resizing a dock

This is a follow up to 60be2fb6a1.

Above change only covers resizes via QMainWindowLayout::hover. Doing a
resize via the separator was not covered.

Task-number: QTBUG-70571
Change-Id: Ic1d1cc6d672b97e8c130864f8708268fd6003204
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Karsten Heimrich <karsten.heimrich@qt.io>

src/widgets/widgets/qdockarealayout.cpp

@@ -3295,6 +3295,8 @@ int QDockAreaLayout::separatorMove(const QList<int> &separator, const QPoint &or
     delta = pick(o, dest - origin);
     delta = separatorMoveHelper(list, sep_index, delta, sep);
 
+    fallbackToSizeHints = false;
+
     if (index == QInternal::LeftDock || index == QInternal::RightDock)
         setGrid(0, &list);
     else