Po.st offers the ability to open your sharing channel login screens and description fields in a pop-up (new window) instead of a new tab.
It is easy to switch between the Pop-up and Tab options in the Social Tools section of your dashboard.
First click on the Update button in the Sharing Buttons box.
Then scroll down the page to find the Advanced Settings.
In the Advanced Settings, you can switch between the Popup and New Tab options in the section marked Share Dialog.