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Pull #116

travis-ci

Fix #118: Change isVisible to check for clickability to improve performance

Compared to a baseline profile[1] of Price Tracker's current 'isVisible' implementation (on which Fathom's 'isVisible' method is based), this clickability approach offers a 53% (146 ms) reduction in Fathom-related jank[2] for the same locally hosted sample page.

This is largely due to removing the use of the 'ancestors' Fathom method in 'isVisible'[3], which was performing a lot of redundant layout accesses (and triggering a lot of layout flushes) for the same elements. Also, at least in an extension application, DOM accesses (e.g. repeatedly getting the next 'parentNode' in 'ancestors') are very expensive due to X-Rays[4].

Notes:
* If the proposal to the W3C CSS Working Group[5] is implemented, this clickability approach could forgo the workaround and see as much as 81% (374 ms) reduction in Fathom-related jank[3].
* This implementation can still benefit from memoization, as the same element (e.g. 'div') could be considered for multiple different 'type's[6].

[1]: https://perfht.ml/30wkWT7
[2]: https://perfht.ml/2Y5FCQ1
[3]: https://github.com/mozilla/price-tracker/issues/319
[4]: https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Xray_vision"
[5]: https://github.com/w3c/csswg-drafts/issues/4122
[6]: https://mozilla.github.io/fathom/glossary.html

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