STEPS TO REPLICATE
Set the default text editor to Atto.  Create an assignment in a course using either the snap or boost theme and submit a submission to trigger the grading access

Grade the assignment and enter long text strings in the feedback comment section
OBSERVED BEHAVIOR:
the box extends beyond the boundaries and the scroll bar is pointless
EXPECTED BEHAVIOR:
It is expected that when using a fixed box the text should wrap to make it readable to the users

ADDITIONAL INVESTIGATION NOTES

we also checked this issue against the boost,clean and snap themes, boost and snap had the same erratic behavior as illustrated in the attached screen shots, clean maintained the fixed text box and increased the horizontal scroll bar but clean wrapped the text down.
In addition we also checked this across browsers in case this was a factor, IE11 FireFox 65 and also chrome 72. There was no noticeable change in behavior so we concluded the browser was not a factor.

Switching to the use of the tinymce editor corrects this issue as it is limited to only Atto

ATTACHMENTS

Screenshot_2019-02-19 Assignment Test new Assignment.png

Screenshot_2019-02-19 Assignment assign_2_1_a.png

pictures of text box extending with text