Keep track of how many tileload/tileunload events are being fired. The counts should always match. See #4093, #4193

Start Load Error Unload Visible Grid load
Grid
Positron

start = unload + visible on screen

(flyTo)
(setView, 5 sec)
(flyTo 20 sec)
(fract. zoom)
(setView, fract. zoom)
(fitBounds)
(image overlay)