macossafari
I am used to of the old private browsing mode in Safari, where you can keep a login session between tabs. But since this new Safari came with Yosemite, the tabs maintain their own sessions.
Is there any way I can keep the login session between safari private tabs till the private window closes, just like how Google Chrome does?
There is no way currently offered by Apple to disable auto-fill based on the browsing mode. However, you could try to switch auto-fill on/off based browsing mode by making a trigger-effect script or macro with AppleScript or an application like Keyboard Maestro.
In Safari 9.0 (El Capitano) you can right-click on the URL while holding down the option key and you then have the option to open the URL in a new private window. This option is provided from regular or private windows, unlike what appears to be the case in previous versions of Safari.
Best Answer
No, this is currently not possible. Safari Private Browsing tabs and windows do not share any session data and are completely independent.
From the Apple Support KB "Safari for Mac: Use Private Browsing windows in Safari":