The blue progress bar is a combination of contacting the server, downloading data, and drawing (rendering) the page in the window. If you click back, depending on your connection speed, connection lag, system speed and a bunch of other factors (how much is happening in the background, how much free RAM you have, etc) it may take a bit for the page to reload in the window, but this does not necessarily mean it is re-downloading all data.
Disabling WiFi (or any internet connection) and trying to go to a URL will ALWAYS fail, because it can't perform the initial steps to determine what page to load (cannot connect to the DNS server). Whether or not Safari ultimately downloads the page again, it will always connect to the website server first to determine if the page changed since the last visit.
You can test if Safari is caching by following these steps:
- Go to Safari menu > Preferences.
- Click on the Advanced tab.
- Check the box for "Show Develop menu in menu bar".
- Close the Preferences window.
- Load this wepage:
http://trakt.tv/user/tubedogg. Once it is fully loaded, click
Reload the current page in the Address Bar.
- Press ⌥+⌘+I to enable the
- Press control+1 to show the
- Click the disclosure triangle next to
- In the right-hand window, review the
Request & Response section. (You may have to scroll down to see it.) Check to see if
Cached says Yes or No.
Note that not all images may be cached, depending on the instructions from the server. For example, some ads will tell your browser not to cache them. But if you review many or all of the images on the page, most of them should say Yes for
Cached. This will determine whether or not Safari is caching correctly.