Shouldn't that be inside the if block? And the handle should be referred to as R.handle instead of parent.querySelector('.resize-handle').

R.handle can be null if resize window and don't touch resize handle. And how you see it inside if block? We don't need to check grid template for that and also we need to have this if-else inside displayResizeHandle

The name displayResizeHandle looks like it shouldn't change anything, I agree. But it already changes style and opacity, maybe it should be renamed

And also maybe I should remove R.handle at all, and init new parent's field inside setup function

Like this:

            if (R.handle != null) {
                R.handle.style.display = "none";
                R.handle.style.opacity = '0';
            }

As I thought, it doesn't work. Also R is global for every parent, it contains handle which is being resized right now

I updated PR, check it. I think it's better now