Thank you for your work. It's great to see RadiAnt getting better and better with each version. Special thanks for the arrow.
I have a couple of questions about MPR.
Is there any way to make custom hotkeys (assigned through Keyboard customization tool) work in MPR mode? And is there any possibility to change default viewport layout in MPR mode?

I also noticed that the arrow tool works strangeley in 2D mode (with only 4 possible directions in a crosslike fasion) when I use it by assigning this function to MouseModifier.Left(rigt, middle - doesn't matter).arrow, while the same arrow picked from the standard tool panel works just fine.

Thank you.

Roman, when customizing keyboard shortcuts, enter "3dmpr" in the top field. You will be able to set shortcuts for 3D MPR window.
The default layout cannot be changed currently.
As for the arrow, I guess your modifier includes the Shift key. This key when pressed makes lines (and arrows too) straight.

I've found this 3dmpr commands, thank you. But the arrow tool is absent in there. There's only "SelectArrow". Is there a simple way to add it?

Actually, 3DMPRWnd.SelectArrow selects the arrow tool. I'm not sure what you exactly want to achieve?

I wanted to assign the "Arrow" tool itself for MPR mode, not the "Select.Arrow". Like in MouseModifiers.LeftButton.Arrow for 2d mode.

Currently mouse modifiers work only in 2D viewer. We will add the support for modifiers in 3D viewers in future versions.