My AC solution fails whenever the left and right sides of a frame are both obscured. E.g.:

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...........┌─────┐................................
...........│.....│................................
....┌──────│.....│..........┌─┐...................
....│......│.....│..........│.│...................
....│......│...┌┐│..........└─┘...................
....│......│...└┘│────┌─────┐.......┌─┐...........
....│......└─────┘....│.....│.......│.│...........
....│......│....┌───┐.│.....│.......└─┘...........
....│......│....│...│.│.....│...┌──┐..............
....└──────┘....│...│.│.....│...│..│..............
................│...│.│.....│...│..│..............
................└───┘─└─────┘...└──┘..............
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