Fix hover out when exiting visible frame.

Summary:
Previously the hover out hitbox was calculated wrong, causing the
hitbox to be too big e.g. for items with fewer actual text lines
(and a frame sized to reflect it) than the configured text space.

Kai mentioned this on IRC.

Reviewers: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4874