Flutter : fix FlexibleSpace position

FlexibleSpace changes its size on scroll. The content inside also changes its position as the scroll slides. This time, I wanted to make the content inside FlexibleSpace to remain in its position, so that when the scrollable content moves, it slides over the flexible space as if sliding door is closing.

FlexibleSpaceBar(
   collapseMode: CollapseMode.none,
)

This could be done with FlexibleSpaceBar and its parameter collapseMode. Setting the collapse mode to none, the flexible space won't move an inch even the scroll slides up or down.