This is a response to an old post however we are seeing the same issue with the RadGrid HeaderContextMenu.
We turned on the RadGrid's HeaderContextMenu so that our users can turn on/off columns on the grid.
The grid happens to be on a RadWindow so the space within the window is rather small and we have about 100 columns that the users can select.
Because of the large number of columns, the popup scrolling window that displays the columns goes off the bottom of the Window.
From the post above I see that we can set the <HeaderContextmenu> <DefaultGroupSettings Height="100"> to shrink the size of the columns popup and that does work to resize that window.
However, as the user above discovered, as soon as you set the DefaultGroupSettings height and mouse over the up and down arrows that allow you to scroll through the columns list, the popup window always disappears after about 1 second.
This is the same bug that the user describes in number 2) above.
I have found this same issue in Chrome and Firefox.
So I am left with 2 unacceptable outcomes:
1. Shrink the columns popup list using the DefaultGroupSettings height property but then the user may not be able to select columns before the popup disappears.
2. Don't set the DefaultGroupSettings height and the user will not be able to see all of the columns...actually it turns out they can use the keyboard arrow keys to scroll down the outer window and then use the mouse to scroll the columns popup but many users cannot grasp this functionality.