In the vim-gtk version of evim, selecting text and then beginning typing produces the normal WYSIWYG result of the first character typed deleting the selection and replacing it with that character (which seems to be stored internally as two separate actions, judging by "undo" results). Or rather, it does so for most characters, including lower-case "q" and capital "S" (just the particular characters I tested trying to narrow this down).
If, however, the character typed is a capital "Q", then it instead leaves the selection intact, but splits the current line (if it is over 79 characters) between words in order to produce a series of lines no longer than 79 characters in length, all whitespace-indented to the same level as the current line. This happens regardless of whether the capital is induced by holding down shift, or by having pressed caps-lock.
Typing a Q without text being selected works normally.
What is going on?