This is as designed. Errors that can be associated with a particular field are put to the right of the field. You can see this if you flip to the plain text editor and leave everything blank. It also happens on various account pages. We don't want to put the errors inline because that would cause page reflows.
(Because of the structure of the WYSIWYG editor, it's not possible to associate an error with that.)
I usually use 1152 x 864 resolution so this is not an issue for me. But for those on the lesser resolutions (eg. 1024 x 768), a part of the error message box overflows the browser viewport and causes a horizontal scrollbar to appear.