Design problem in ListView update code

[ogoffart]

In the ListView, some stale state might be visible (scrollbar state) when the model change.

The ListView will set the size of the viewport from the update code. ensure_updated_listview which is called from the visitor that visits the tree for the painting. The problem is that the viewport height is used to compute the scrollbar size.
Which means that we don't repaint the scene if we change the viewport size while painting. Also the poition of the viewport might also change from ensure_updated_listview

The symptom of that is that the scrollbar size are not updated when the model change with the native style.
This was reported in slint-ui/cargo-ui#10
(The fluent style is fine because it draws the scrollbar after the flickable)

We could work-around that problem by forcing the scrollbar to be painted after the Flickable, but this is not how the native style work because it draws the frame and the scrollbar in one go. We eventually need to split that anyway to support styles that draw the scrollbar over the content only when scrolling.

Maybe a definitive fix would be to change the binding on the viewport-height so it depends on the model via a runtime function.