Originally Posted by corporate-wage-slave

The policy is that it's under the control of the passenger. If the crew dim over all windows, then they should then release the hold and thus allow you to clear the shade. There are occasional reports of cabin crew not doing this, but I've never experienced it and I've got a 3 digit number of flights on BA's 787s, so I think it's a sub 1% chance. I can imagine longer trips (LAX/SFO/SAN) this may be a greater risk.