Selection inset varies by document scroll position

[hubgit]

Describe the issue

There seems to be a discrepancy between the way the selection is drawn at the top of a document and further down the document (once the top of the document is scrolled out of the top of the viewport?)

In the linked demo, making a selection at the top of the document is consistently indented:

When making a selection further down the document, the edges of the selection extend to the edges of the content box:

Browser and platform

Chrome, Firefox and Safari on macOS

Reproduction link

https://codemirror.net/try/?c=aW1wb3J0IHtiYXNpY1NldHVwLCBFZGl0b3JWaWV3fSBmcm9tICJjb2RlbWlycm9yIgppbXBvcnQge2phdmFzY3JpcHR9IGZyb20gIkBjb2RlbWlycm9yL2xhbmctamF2YXNjcmlwdCIKCm5ldyBFZGl0b3JWaWV3KHsKICBkb2M6ICJmb287XG4iLnJlcGVhdCgyMDApLAogIGV4dGVuc2lvbnM6IFtiYXNpY1NldHVwLCBqYXZhc2NyaXB0KCldLAogIHBhcmVudDogZG9jdW1lbnQuYm9keQp9KQo=

[marijnh]

Attached patch should help.