I'm experiencing the same problem. It captures the visible part of the window and doesn't auto scroll. Command according to menu should be Ctrl-Shift-PrtScreen. Doing that indeed a red box appears in the upper-left, with instruction to use Ctrl-Shift-click. Doing that, it says ""region 1282x828"(the size of the visible window), to capture press click. Result: as mentioned above. Using latest version 2.56.01 and trying to capture an Internet page. I'm using Google Chrome. Same behaviour in IE by the way.
Another problem encountered: after doing the (partial) capture the capture mode remains active. When checking the result in Screenshot Captor program window, the red outline remains active and tries to capture the result in Screenshot Captor (...). And even now, when typing this message, it still tries to capture this forum page. Etc. I don' t have a solution, but maybe this "user input" can help make it a better software.