Fix multiline selection

In qt6, QRect::normalized changed it's meaning and it now has a 'span'
that preserves the old meaning.

Use the old span to get the old version of normalized so we can match on
the corners once again.

see https://codereview.qt-project.org/c/qt/qtbase/+/309932 for details.

BUG: 482249
(cherry picked from commit 717751a1)