@mihael I have this problem as well, and it's because the X11 server doesn't really know what the keyboard height is. The responsive patch for Whatsapp Web/Signal/etc is basically shrinking the window size inside the X server to the standard keyboard height when the keyboard is out. But it would need a configurable option to change that value in the app itself, I think. Unless there is some magic (and preferably confined) way to yoink the keyboard height from the keyboard?
Personally, I think the keyboard height in UT is way too low. But, an adjustable height keyboard Should Just Work once Mir 2.x comes out... right?