Canvas supports automatic grading periods — that is, assignments are automatically sorted into their proper window based on their due date.
That said, it can be disconcerting, as the window changes, to see vast swathes of assignments disappear (or appear). Students can change which window (or grading period) they are viewing in a particular course’s Grades view by using the drop-down menu above their own name:
Why can’t I always see the Total grade?
Total grades are turned off by default. Your teacher may (or may not) decide to enable visible total grade calculations for your class based on their particular grading philosophy. A reasonably canny student with a calculator can probably figure out an approximate total grade with ease.
What does the Total reflect?
The total (if visible) reflects the total grade for the currently selected grading period. That is, if you are looking at “Window 1,” then then Total that you see is calculated for Window 1. Alternatively, if you are looking at “All Grading Periods,” then the total you see is calculated for the entire year.
The assignment group averages are similarly calculated based on the visible assignments (i.e. the selected grading period) and, if assignment groups are weighted, that weighting is factored into the total grade for the visible grading period/assignments.