When map div has content at init, layer mouseenter zone gets offset

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- We have a map div which has some noscript text at init ("JavaScript must be enabled")
- We create a MapBoxGL JS map in that div, overwriting the noscript
- We add a GeoJSON as a circle layer
- We use a mouseenter event to trigger a popup as recommended in the docs

In MapBoxGL 1.9.0, I couldn't hover over the marker; the mouseenter/mouseover zone is shifted down ~15 pixels, apparently due to that initial noscript text.
I have a demo here: https://mggg.github.io/covid-flows/index_mb.html?4

Before trying removing that noscript, I downgraded to 1.8.0 and the map worked again. It did not assume an empty div or remove the noscript text. So 1.9.0 appears to be handling these divs differently.

Start with the linked covid-flows demo and reproduce the hover offset in MapBoxGL 1.9.0, then compare it with 1.8.0 using a map div containing the reported noscript text. Trace the container and layer mouseenter behavior to identify the regression; done means the hover zone aligns with the circle layer without requiring an empty initial div.

Written by the indexing model from the issue text.