With both 5.8 and the current dev HEAD, when e.g. using the "view page source" action in the context menu on a page loaded via a POST, the page is reloaded. This works correctly in Chromium.
This can be reproduced by filling out something on http://httpbin.org/forms/post, submitting it and then viewing the source of the JSON. In the demobrowser, this leads to a HTTP 405 because the page is reloaded with a GET; in Chromium the source is shown correctly.