Please refer to the attached sample project and follow the steps from the gif file. The scrollbar needs to be in the middle.
Note: the issue can be reproduced without action list as well.
Add a RadSpellChecker, a MS TextBox and a MS RichTextBox to the form. Try to set the AutoSpellCheckControl to RichTextBox, but the control is missing in the drop down:
Workaround:
private void radChat1_ItemFormatting(object sender, ChatItemElementEventArgs e)