Before using the mesh, you need to use the container first.
You can use .container if you want your lines to have a fixed width or .container-fluid if you want your lines to be full width.
For your code in the script, replace the first .row class .row one of the two classes mentioned:
Option 1: .container
<div class="container" style="background-color: lightblue"> <div class="row"> </div> <div class="row"> </div> <div> </div>
See in action in the fiddle .
Option 2: .container-fluid
<div class="container-fluid" style="background-color: lightblue"> <div class="row"> </div> <div class="row"> </div> <div> </div>
See in action in the fiddle .
See also : Grid System (Bootstrap 4 Documentation)
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