R1:8025bc6432e2

Keep hovercards on screen a little harder

Summary: Ref T8980. This calculation was not quite right and you could sneak one off screen if you tried carefully.

Test Plan: Couldn't sneak one off screen anymore.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8980

Differential Revision: https://secure.phabricator.com/D14880