Hi Jon,
Thank you for iterating super fast!... I've done further elaborate tests as reported below, on three separate WP 3.9.1 sites, each with a different caching plugin.
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TEST 1 - Staging site with WP Fastest Cache turned on:
- Testing is done by first clearing the cache in Wordpress, and clearing history in Firefox and Opera. Firefox is used to open standard URL, and Opera is used to open URL appended with "?theme=fastr-child" string. On both browser, the same clicking sequence was perfomed.
- In Opera, the sticky "?theme=fastr-child" settings successfully appended random keyword=value strings in subsequent URLs and shows the alternate theme.
- Afterward, in Firefox, opening the URLs following the same sequence (but without the "?theme-fastr-child" string on the first URL opened), showed the pages with the default theme.
- The exception is when a particular page has been opened first without following the above sequence, i.e. if a page was first opened without the "?theme=fastr-child" string or the random "?keyword=value" string (in Firefox), and thus, gets cached with the default theme, the sticky "?keyword=value" string loses it's magic in Opera (it doesn't append the URL with the random "?keyword=value" string).
- e.g. http://theastro.staging.wpengine.com/astrology-glossary/cancer-622-722/ and http://theastro.staging.wpengine.com/astrology-glossary/cancer-622-722/?theme=fastr-child
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Test 2 - WPEngine site with it's native caching mechanism:
- Cache was cleared, and Firefox + Opera is being used after all history have been erased. Firefox is used to open default URL / theme (no "?theme=2012" string), while Opera is used to load alternate URL / theme (with "?theme=2012" string).
- Alternate theme was successfully loaded only on the first URL with the "?theme=2012" string, but not on subsequent pages clicked. On those subsequent pages/URLs, the random "?keyword=value" string was not present either.
- Every time any page is loaded without "?theme=2012", default theme is loaded, and with "?theme=2012", the alternate theme was loaded. But stickyness of the alternate theme is not working.
e.g. http://blog.theastrologer.com/astrology-glossary/scorpio-1023-1122/ and http://blog.theastrologer.com/astrology-glossary/scorpio-1023-1122/?theme=2012
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Test 3 - A test site with the popular W3 Total Cache plugin
- Similar setup as the above tests, in terms of browser and alternate theme's URL string.
- In W3 Total Cache, I enabled page, database, object and browser caching with default settings (via 'General' tab)
- The results with Firefox and Opera is similar to Test 1 using WP Fastest Cache.
- There is a setting in W3 Total Cache if Page caching uses Disk (Basic), it's possible to "Cache URIs with query string variables"... but the result is still the same. This is tested with a different set of pages than the one shown below, to ensure those pages have not been cached (cache was also cleared after changing the settings.)
e.g. http://bowo.io/playground/wp/markup-text-alignment/ and http://bowo.io/playground/wp/markup-text-alignment/?theme=2012
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Conclusion: seems like this is a partially working solution that will not work where pages have been cached beforehand or have been visited by other users (without the alternate theme's URL string), which probably would be the common case with cached sites.
Let me know if you need me to do certain/other tests, or if you need anything from us to do some testing on your own.
Many thanks!!!