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Looking at this code https://github.com/yui/yui3/blob/073552364face4aa98b01feb97c97cb50b33440a/src/widget-position-constrain/js/Widget-PositionConstrain.js#L76
I cannot see a good reason why "lcrc": 1, "rclc": 1 is missing from the x array, and "tcbc": 1, "bctc": 1 is missing from the y array.
And, that lack is causing this problem in Moodle: https://tracker.moodle.org/browse/MDL-44363.
Also, there are still more combinations of alignment that can potentially lead to overlap. E.g. tlbr will cause overlap if the thing you are positioning relative to is very close to a corner of the containing element.
Of course, if there is a good reason why we can't change this in YUI, there is a work-around for Moodle, to change the alignment we use to one that supports preventOverlap, even if that looks slightly less nice, in our opinion.
Thank you for your consideration.