screen does not scrolling when moving cursor in function

I am not very sure whether this is a bug of screen update.

OR some kind of rule for the vim, sorry i can't find this kind of rule mentioned in the help doc.

It happens when i do some kind of moving cursor action in a vim function which is used to

move the cursor position (out of screen) in another vim window.

So you can reproduce this using the script i attached.

The step is :

1. start a new vim and source the script

2. you can see that the line in another window do NOT scroll to the line could show the cursor

3. CTRL-W CTRL-W ( move the cursor into another window )

    Now the screen update to let the cursor line show.

However, when i focus to the below window and run command 

:call cursor( 1, 1 )

or

:call cursor( 99 , 1 )

The screen will update immediately to the line i set to cursor function.

Now, we can find that, the same call to cursor() in the MoveCursor() function 

will NOT update the screen. Does this a problem?

Or is there some option could preventing is ?

PS: i try to "set nolazyredraw", but it seems not work for me.