Two lessons from the thumbnail work, the second of which nearly shipped.
We parked progressive loading on a count of images and the cost was in bytes.
'Measure the real booth before optimising it' was followed and still gave the
wrong answer, because we measured the dimension that was easy to measure rather
than the one the user feels.
And a cache living inside the thing it describes can age that thing. Excluding
every path under the cache dir passed its own test and was still wrong: creating
the directory touches the BOOTH's own mtime, which is what _newest_mtime seeds
from. The contents were excluded; the existence was the leak. Had it reached the
Desk, one index load would have pushed every booth's expiry out and the TTL
would never have fired again.
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