wrong copy and paste when using code editor

1. write any text  eg. "test!!!" somewhere for example Windows text editor
copy the text you have written eg. using ctrl+c

2. open a QML file
eg. 

import QtQuick 2.15
import QtQuick.Controls 2.15
Item {
    width: 1920
    height: 1080
} 

3. select all contents of the QML file using ctrl+a
4. paste the copied text
expect QML become 

test!!!

but not, the old text didn't be overwritten

import QtQuick 2.15 
import QtQuick.Controls 2.15 
Item {
     width: 1920
     height: 1080 
}test!!!