One more thing! I've found this is indeed a bug with the "radeon" driver, meaning this bug is the right section. I've rolled back to and built the revision 1cf7a5494fa94e8d9f30f9b2905dfbe6d4faa445, "Fix pasto in connector table setup for vga powerbooks," and the bug went away! That means that this bug must have been introduced in the flurry of changes on Monday, August 25 (UTC).
One more thing! I've found this is indeed a bug with the "radeon" driver, meaning this bug is the right section. I've rolled back to and built the revision 1cf7a5494fa94e8 d9f30f9b2905dfb e6d4faa445, "Fix pasto in connector table setup for vga powerbooks," and the bug went away! That means that this bug must have been introduced in the flurry of changes on Monday, August 25 (UTC).