Why Left Never Backs Up

A sliding window is correct only when the left pointer never needs to move backwards. Stated as a property of the predicate that decides whether a window is acceptable, what is that requirement?

The answer must be a contiguous subarray rather than a subsequence

If a window is unacceptable, every window that contains it is also unacceptable

The array values must all be positive

The array must be sorted, so that the window sum grows with the right endpoint