Hello,
I don't know if this may be useful to anybody else or not, but just in case so that you don't waste as much time as I did finding the solution:
-We have a theme which is not based on bootstrap although it is responsive (up to a point; it is in our 'To Do' list to revamp the theme).
-I installed the Kaltura Video package v4 for Moodle (incidentally: the Video assignment activity is just fantastic for assessing the interview skills of our students!).
-With our theme, when tutors were marking student by student, the page for 'single_submission.php' was bigger than the screen size and even zooming the browser in, the 'Feedback' box was bigger than the screen width.
-In the file 'moodle\mod\kalvidassign\styles.css
If you replace the line:
#page-mod-kalvidassign-single_submission #page #page-content #region-main-box {
left:1px;
width:180%
}
With 'width:100%', that fixes the problem.
Funny enough, single_submission.php is displayed correctly for those themes which use bootstrap, such as 'clean'.
Regards,
Mari Cruz