Spawned off from bug 1144096 comment 28 n = number of child frames (both grid items and grid-aligned abs.pos. descendants) dholbert suggested a hash map from frame pointers to mGridItemInfo array indices to make it O(n). Another way would be to replicate the GridArea on each fragment (frame property), but then we'd need to update these copies when the area changes.

Note also bug 1306705 which is similar.