TextEdit uses the default cocoa text widget. Both TextMate and Safari have reimplemented it. TextMate’s text widget is substantially more sophisticated, and Safari’s is mostly compatible except for a few annoying edge cases (it’s been a few years since I investigated; maybe some have been fixed).
Amusingly enough, Mail’s compose window uses WebKit for the content area. According to http://webkit.org/b/73888 the issue is specific to the WebKit2 layer that is currently used only by Safari.
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Hmm, I can reproduce this in Safari, but not in TextEdit, nor TextMate, under 10.7.2.
TextEdit uses the default cocoa text widget. Both TextMate and Safari have reimplemented it. TextMate’s text widget is substantially more sophisticated, and Safari’s is mostly compatible except for a few annoying edge cases (it’s been a few years since I investigated; maybe some have been fixed).
As mentioned in the link's comment thread, ^Y is broken for Webkit. I just tried this in Mail.app and it worked fine.
Amusingly enough, Mail’s compose window uses WebKit for the content area. According to http://webkit.org/b/73888 the issue is specific to the WebKit2 layer that is currently used only by Safari.