how can i get the size of a soft kayboard working with javascript and jquery?

I have a big problem with this, I have been working with a bewsite that contains an iframe, I had no problems until i tried to test it into my mobile phone.

I have 1 iframe with 3 divs, all of them with position relative (if i change it they go out of my mobile). 1 of them has an input, and when i click on it, the soft keyboard appears. My screen does not resize, so I only am able to watch 1 of the 3 divs. I have been reading about this, and i have a solution that i do not know how to programme.

Here: Get viewport height when soft keyboard is on

I found this: This solution is a bit convoluted, but it might work...

Detect whether or not the keyboard is active. Grab the viewport width/height Subtract the height of the keyboard.

I know when the keyboard is active, because is when i click on the input, but i am not able to get the size of the soft keyboard.

If someone could help me I would greatly appreciate it.

(it is my first time writing here, so if you need more information just let me know)