Assistant: keep text at the top visible when resizing

After following a link to look up a keyword, resizing the width of the
window would lead to the looked-up position being scroled away.  This
fix is adapted from the corresponding fix in Qt Creator,
https://codereview.qt-project.org/c/qt-creator/qt-creator/+/274996



Fixes: QTBUG-34525
Change-Id: Ia9c2aab4b2c8039965eff098ff51dbdcc4967a01
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
(cherry picked from commit b3cdd63d)