Using diff -> unified on a file with 10000 lines has a different look

If a file has changes past line 10000 / is more than 10000 lines long the CSS does not render the background in a different color on changed lines, but changes the font color instead.

In https://codereview.qt-project.org/#patch,unified,25848,1,src/widgets/kernel/qwidget.cpp it is visible that the background of the changes does not change, but the font color.