Large source segments can be scrolled trough when they exceed the size of the preview window, but large target segments remain fixed and parts that exceed the window size can't be accessed; the selection in the example image shows where the text is extended past the viewable area.
Right, scrollbar is missing on larger segments. You may try version in Git repository. Must be checked out and compiled.
If you cannot do that then cannot me through this project or OmegaT+. I will see if I can get something prepared for you.