The HTTP Pragma header is obsolete for HTTP 1.1,
and technically only defined for a HTTP request (not response).
Refer to https://www.mnot.net/cache_docs/#PRAGMA
,http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.32.
It is superseded by the "Cache-Control" directive.
See HTTP 1.1 spec at https://tools.ietf.org/html/rfc7234#section-5.4:
'Because the meaning of "Pragma: no-cache" in responses is
not specified, it does not provide a reliable replacement for
"Cache-Control: no-cache" in them.'
Sending a "Pragma: nocache" response header is a prudent
backwards compatibility measure for HTTP 1.0 clients.
The intended behaviour is for the majority clients as well as any
intermediary proxies to ignore this header.
Sending an empty Pragma is a known hack
for preventing PHP from adding "Pragma: nocache" to responses
with started sessions (see http://php.net/session_cache_limiter),
since PHP does not allow unsetting existing header() calls.
In IE11 windows 8 call to window.localStorage was throwing out an access denied error. Using try and catch manages the issue and allows the script to execute in IE 11 in desktop mode.
I think it is a problem with IE11 rather than the way Silverstripe is implementing the preview via an iframe from what I have been reading. http://blogs.msdn.com/b/ieinternals/archive/2009/09/16/bugs-in-ie8-support-for-html5-postmessage-sessionstorage-and-localstorage.aspx. It seems that the way IE11 deals with localStorage is broken in certain cases but I am not 100% certain of the cause yet as I have not been able to find a definitive answer. I only noticed it was a problem when a new client said they couldn't view the admin screen properly in IE11. I took a look in IE11 and I was experiencing the same problem which makes the admin interface layout screw up and the preview doesn't work due the error mentioned in the first post.
Instead of the original code I submitted I have amended it and added an additional function to test more robustly to see if localStorage is available and can be accessed properly. It is a copy of the code on a blog post Mathias Bynens has written about detecting if localStorage is available and can be used: https://mathiasbynens.be/notes/localstorage-pattern
I have added a console.warn as you suggested if localStorage is not available so that at least you get a warning if localStorage tests fail.
I have tested this on Windows 8.1: Firefox, Chrome & Mac: Firefox, Safari, Chrome and it seems to work as expected. On IE11 it displays the admin area correctly now but obviously doesn't save the preview settings between page loads if localStorage is not available.
