Hi calaban9
Welcome to Apple Discussions
Turn off Mirroring!
If you use Extended Desktop you can choose a resolution for each display, that should not change when the external is switched off.
http://docs.info.apple.com/article.html?artnum=303086
Resolutions
+When you connect the adapter to the computer, the screen may refresh momentarily as the computer prepares itself for the new video output. When the computer is mirroring video, it tries to synchronize the resolution of the built-in and external displays. If they can't be matched, the computer will change the resolution of both screens to the highest common resolution which may cause areas of the built-in display to be blacked out.+
Dennis