fixes https://github.com/flutter/flutter/issues/151854fixes#139113
The regression test uses nested `ListView` because I could not reproduce the issue without using nested `Scrollable`.
The issue is masked when there is only one `Scrollable` outside `DropdownMenu`. While `Scrollable.ensureVisible` can find all the `Scrollable`, the actual scrolling is performed by `ScrollPosition.ensureVisible` , which uses the `RenderObject` tree to find nearest Viewport but it could not find one due to `OverlayPortal` putting the target `RenderObject` at different point. So no scrolling will occur.
However when there are nested `Scrollable`, `Scrollable.ensureVisible` can scroll the outside `Scrollable` normally since no `RenderObject` tree gap exist between the two `Scrollable`
8e50a17d94