When using both the DisallowOverscroll property and WkWebview, there seems to be an issue when scrolling and repainting fixed elements.
When you scroll to the end of the page (where it would bounce if you don't set the DisallowOverscroll), fixed positioned elements seem to bounce but never return to their original position. Visually there is no issue but when you try to click something inside the fixed element (for example a button inside a fixed footer) the click action will not be triggered. Also when you hover the node in the Safari Debug Tools it thinks the element is displayed somewhere else.
I've attached a screenshot and some code to make it a bit more clear.