Delete key not working if at end of line past a space

Problem in 4.0.1 (iframe version):

If I position the cursor at the end of one of the lines of text in the editor, past the space that follows the last word on that line, and then hit the delete key, nothing happens.  If I move the cursor to the end of that word, but before the space, the delete key will delete subsequent text, including spaces, for as long as I hold it down.

Any thoughts on what I may be doing to create this problem (as opposed to a bug in the editor).

Thanks.