So I now know, having belatedly had a look. Thanks.

See subject.  I find that when I click send the spell checker  window seems to have a will of itself.  For a while it appeared half off my computer screen, forcing me to drag it back to somewhere sensible.  Then when I start it it goes somewhere else.  Over time it moves around for no discernible reason. Not a show stopper, but a nuisance.

You have no control over the spell checker window location.  The window position changes depending on whether the misspelling is detected in the header or in the body.  The location of the new message window also affects its location.  A new message window low on the screen can result in the bottom of the spell checker window being off screen.  A search of the forum should show numerous discussions about this.