Creates a swipe-to-reveal header in a ScrollView. The header is visible both when the user scrolls down and when the ScrollView is scrolled to the top.
A SwipeGesture is added directly in a ScrollView to provide an active drawer for the header. This works because the implicit gesture controlling the scrolling and the SwipeGesture are both on the same node and are allowed to share input.
A SwipingAnimation, named swipeAnim, is added to monitor the gesture. This is normally where the reveal part oft he gesture would be done, but it won't be enough, so we leave it as an empty gesture. We want the header to be visible also if the user scrolls to the top without invoking the gesture, such as by pressing the scroll to top button.
We add a ScrollingAnimation, named scrollAnim, that covers the region occupied by the header view. It again is left empty. Ttriggers have progress even if they don't contain any animators or actions.
The headerPanel has a Translation controlling whether it is revealed or hidden. We want this to be revealed when the users swipes down or when the ScrollView is at the top. We use a max expression that takes the higher progress of swipeAnim and scrollAnim.
At the top of the StackPanel we have a Panel reserving height for the headerPanel. It uses an expression to assign itself height, leaving a bit of space for the divider.
By reserving the space we simplify the task of hiding/revealing the header. It allows the small headerPanel to be an overlay on the ScrollView at any scroll position, but also to appear above the scroll contents when scrolled to the top.