The Fixed Window Slide

This animation runs a maximum-sum search with a translucent rectangle exactly k cells wide sitting over the array. Each step, the rectangle shifts right by one cell: the leaving cell flies out with a "−value" label, the entering cell flies in with a "+value" label, a running-sum counter ticks to its new total, and a best-so-far marker snaps onto any window that sets a record.

What to watch for:

• Only two cells ever change hands. However wide the window, each slide touches exactly one leaver and one enterer — that's the O(1)-per-step update that beats re-summing all k cells.
• Which cell leaves. When the cell at index right enters, the cell at right − k exits. Watch the gap between them stay exactly k — this is the off-by-one that bites in real code.
• The counter never resets. The sum is carried forward and adjusted, never rebuilt. If you catch yourself mentally re-adding the whole window, the animation is showing you the cheaper habit.

By the final slide you should be able to predict the next sum before the counter ticks.