See the attached testcase
editing/pasteboard/8145-1.html will illustrate this bug
Created attachment 7568 [details] patch
Comment on attachment 7568 [details] patch r=me
I wonder if this will also fix: http://bugzilla.opendarwin.org/show_bug.cgi?id=8185 REGRESSION (NativeTextField):With active selection in field, copied text pastes with ending line break into native text field