Treat scrollbar as overlay

Summary: This enables text below scrollbar.

Test Plan:
The new trial: {F5912720}

The current Master: {F5912721}

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: Fuchs, ngraham, abetts, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D13543