What needs to be fixed

There is a 15px margin at the top of the first element in the component area. If there are left sidebar modules (whether there are right sidebars or not), then it is added to the top of the sidebar-left-container using the class .sidebar-left-container:

If there is no sidebar-left module, but a sidebar-right module, the margin is added to the top of component area with the class container-component:

Why this should be fixed

It would be tidy

How would you fix it

Inspecting the code in Firefox shows that this code is the culprit:

.site-grid .container-header + div:not(.container-banner) { margin-top: 15px; }

Changing the margin to 0 does the trick:

I think this code is in (templates/cassiopeia/scss/blocks/_css-grid.scss) lines 64 to 71:

(sorry - that is an image. I don't know how to 'quote' from a file other than using the code tags in the editor.)

If that is the correct code, would it affect anything else if it was removed?

Side Effects expected