If you open a message viewer window, then zoom in a bit (using cmd+scroll), then if you click in a portion of the message window the zoomed area (your "window") moves to the top of the message viewer window.
1. Open a message viewer window in Mail
2. Make the window fairly tall
3. Select a message with a lot of text, such that it reaches to the bottom of the window.
3. Zoom in (using cmd+scroll wheel) a bit, maybe halfway or more
4. Move the zoomed area to the lower portion of the screen, still over the message viewer window
5. Click in the message window on some text
The zoomed area moved to the top of the screen.
Created attachment 25523 [details]
patch to fix incorrect zoom bounds