A few `AutoScrollToSelectedItem` improvements:
- Scroll to current selected item when it's set to true
- Scroll to current selected item when list first displayed
- Scroll to current selected item when attached to visual tree if the selection was changed while it wasn't attached
Fixes#4100
- Remove `SelectedItemsSync` and store `SelectedItems` in a new `InternalSelectionModel`
- Store transient `SelectingItemsControl` state in an `UpdateState` object
Fixes#4272
Previously, `UnmanagedMethods.ShowWindow` wasn't called when trying to show a window with `WindowState="FullScreen"`.
This is because win32 conflates showing a window and setting a window state, and setting all windows states except fullscreen require a call to `ShowWindow`, except fullscreen which does *not*. However it does require a call to `ShowWindow` when we are actually showing the window ;)
Clean up the `EffectiveViewportChanged` subscriptions before calling `OnDetachedFromVisualTree` so that (un)subscribing to the `EffectiveViewportChanged` event in `OnDetachedFromVisualTree` doesn't cause a leak.
Using `SetValue` with `Style` priority caused a new entry to be added to the `IsVisible` priority store each time the value was updated, causing a leak. Setting it with local value priority subtly changes its semantics but hopefully not so that anyone notices.
#4577 was caused because the `sender` of the `CollectionChanged` event didn't correspond to the collection that the event listener was registered on. Instead create an `Entry` object for each collection and put the event handler on that.
Steven Kirk
Maksym Katsydan
Dan Walmsley
Benedikt Schroeder
Kir-Antipov
Luis von der Eltz
ShadowDancer
amwx
Nikita Tsukanov
Jumar Macato