ToanLS,
Thank you for your high level reply 
,
that’s what I want to know.
From the programmer’s standpoint,
restrictions before input are the simplest and most effective.
with my idea, I could replace “5|0” with “|0” to avoid over range,
where “|” stand for user’s input (if it is possible to over my range).
About the user’s experience, Yes,
because the text box control is a beginner’s level practice,
so I am surprise to see this on a “leading software”.
Also, I’ve post two more article about UI design
-
Apply deep background color for selected pane.
(I mean the selected icon’s background color not the pane) - Place new Text element on a “immediately visible area”.
They all mean the same thing, “let user know where they are”.
Regards