ScrollViewer's Viewport Height VS Actual Height

Question

Both are quite general terms but I'm curious to know when these height will be different apart from the case we're using Virtualization?

One more question: I read on MSDN:

If CanContentScroll is true, the values of the ExtentHeight, ScrollableHeight, ViewportHeight, and VerticalOffset properties are number of items. If CanContentScroll is false, the values of these properties are Device Independent Pixels.

However I'm facing an issue with ViewPort Height: I've 2 listbox in application:
1. Which have Virtualization Enabled and CanContentScroll = True.
2. Which have no virtualization and CanContentScroll = True.

In ListBox 1 while drag-drop Viewport Height comes to 4/5 (Number of elements currently visible). However in ListBox 2 i get Viewport Height equal to Actual Height of Listbox.

Why this difference?

Few more findings:
1. Scrollable Height is number of items not visible in scrollviewer
2. Viewport Height is number of items visible in scrollviewer.
Thus Viewport Height + ScrollableHeight = Extent Height

Can someone please explain what's the difference between two listboxes? I need ViewPort hieght in case of Listbox 1

Answer 1

the ActualHeight is the actual height of the ScrollViewer. The Viewport is what is visible from the ScrollViewers Content. So to answer your question: ViewportHeight differs from ActualHeight if the horizontal Scrollbar is visible by the Height of the Scrollbar.

so, to sum this up:

ActualHeight = ViewportHeight + HorizontalScrollbarHeight