Thanks Jon!
I've deleted all plugin files, cookies and cache and installed the Proof of Concept version on all three WP environments I used in previous testing. Here are the results:
1. WPEngine staging site using WP Fastest Cache
2. MediaTemple site using W3 Total Cache
3. WPEngine production site using their custom caching system
All three sites have the TwentyTwelve v1.4 theme set to 'Sticky' via 'theme=2012' query string. First step on all three environment is to open homepage with no query string, thus, loading default theme.
1. WPEngine staging site using WP Fastest Cache
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- Plugin seems to be working nicely. On first load of a page with query string, http://theastro.staging.wpengine.com/the-astrologer-daily-june-30th-2014-2/?theme=2012 all the links are properly appended with the query string.
- Subsequent click also appends query string nicely. This is 'Leo' in first paragraph: http://theastro.staging.wpengine.com/glossary/leo-723-822/?theme=2012
- From homepage with default theme, clicking a link to a post displays that post with alternate theme (2012), though there is no appended query string in the URL: http://theastro.staging.wpengine.com/the-astrologer-daily-june-28th-2014-2/ ...indicating the cookie is working it's magic... intentionally?
2. MediaTemple site using W3 Total Cache
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- Plugin seems to be working nicely. On first load of a page with query string, http://blog.theastrologer.com/the-astrologer-daily-june-30th-2014-2/?theme=2012 all the links are properly appended with the query string.
- Subsequent click also appends query string nicely. This is 'Markup: Image alignment' in the sidebar: http://bowo.io/playground/wp/markup-image-alignment/?theme=2012
- From homepage with default theme, clicking a link to a post displays that post with alternate theme (2012), though there is no appended query string in the URL: http://bowo.io/playground/wp/title-with-special-characters/ ...indicating the cookie is working it's magic... intentionally?
3. WPEngine production site using their custom caching system
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- Plugin doesn't seem to be working all the way. Page with query string http://blog.theastrologer.com/the-astrologer-daily-june-30th-2014-2/?theme=2012 is loaded with the TwentyTwelve theme, but no trace of appended string on links in the page.
- Subsequent click on 'Leo' in first paragraph takes me to http://blog.theastrologer.com/astrology-glossary/leo-723-822/ with no appended query string, thus, displaying default theme.
- Clicking another post from the homepage with default theme takes me to that post with default theme (no query string in URL): http://blog.theastrologer.com/the-astrologer-daily-june-28th-2014-2/
Conclusions
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It seems that for sites with standard WP configuration and caching plugin, the Proof of Concept is working nicely. But WPEngine might have altered their WP configuration and has a special caching mechanism that renders the Proof of Concept inert, unable to perform it's intended purpose. Or, there might be a conflict with another plugin.
So, assuming you might need to go inside the WPEngine installation to figure things out... I've created an SFTP user and admin user in WP for you and emailed the info.
Looking forward to hear what you have to say about the results above.
Thanks!
Bowo