The server may set this optional cookie to echo the HTTP method of the initial request.
If the initial page was loaded with a non-GET HTTP method, Unpoly prefers to make a full
page load when you try to update a fragment. Once the next page was loaded with a GET method,
Unpoly will again update fragments.
This fixes two edge cases you might or might not care about:
- Unpoly replaces the initial page state so it can later restore it when the user
goes back to that initial URL. However, if the initial request was a POST,
Unpoly will wrongly assume that it can restore the state by reloading with GET.
- Some browsers have a bug where the initial request method is used for all
subsequently pushed states. That means if the user reloads the page on a later
GET state, the browser will wrongly attempt a POST request.
This issue affects Safari 9-12 (last tested in 2019-03).
Modern Firefoxes, Chromes and IE10+ don't have this behavior.
In order to allow Unpoly to detect the HTTP method of the initial page load,
the server must set a cookie:
Set-Cookie: _up_method=POST
When Unpoly boots it will look for this cookie and configure itself accordingly.
The cookie is then deleted in order to not affect following requests.
The simplest implementation is to set this cookie for every request that is neither
GET nor an Unpoly request. For all other requests
an existing _up_method cookie should be deleted.
