Comment 4 for bug 1733123

Selection is limited by screen size and it seems screen size is determined incorrectly.

It could help to understand this better if you run attached regiongrabber_dbg, "Take screenshot" and copy log from the main window.
Ready-made 64-bit binary is included, but you can also build it from source:
1. Open Software & Updates and enable "Source code"
2. Run in terminal:
sudo apt-get build-dep basket
cd regiongrabber_dbg
qmake && make