GridViewDataColumn doesn't respect FallbackValue when the source property returns DependencyProperty.UnsetValue

Implement Excel-like horizontal locked scrolling. Clicking on the scrollbar should scroll into view the first column that is not visible at the moment, i.e. all visible columns should be moved out of the viewport.

Enhance aggregate functions (like Sum) in footers so they support expression columns in RadGridView. 

See the attached project. It works for DataColumn, but raises an exception that no suitable generic Sum method could be found when I set the Sum footer for an expression column.

The same works fine with the old Nested mode.

It's so strange that Telerik's components don't support such basic mechanics of WPF...

When TabStopMode is set to "Skip" on all columns and you press Tab, RadGridView scrolls a little.

Workaround: define a custom KeyboardCommandProvider and clear the pending commands.

The group panel item image of a column with custom header is blurred

When the ScrollMode is set to Deferred, then only the vertical ScrollBar shows a position indicator. The same is requested on horizontal scrolling.