I have several textboxes on some page.
HI - The UltraTextEditor does have a mouse down event - client-side
You can call the spellchecker on the clientside as well.
Here is a help link to the Webspellchecker Client-side object
https://es.infragistics.com/help/aspnet/infragistics.webui.webhtmleditor~infragistics.webui.webhtmleditor.webhtmleditor~enablespellcheck