So far, we didn't properly disable layers that are outside the zoom range. This means that we rendered layers that should not have been rendered, albeit we didn't make any attempt to load tiles for those layers. However, when zooming in/out, existing tiles might already have been loaded in the source which continued to be rendered. In most cases they weren't actually visible because either the matrices weren't updated, or the clip IDs weren't set so that they would be "rendered" off-screen and clipped completely. In any case, we did way too much work.
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