The grid editor doesnt open correctly if an actual editor field component is set.
Although the grid editor panel opens fine in the tests, it seems that is only the case because the tests aren't actually setting field components for the editor. If you actually set a component via:
grid.addColumn(Person::getName) .setCaption("Name 1") .setEditorComponent(new TextField(),Person::setName) ;
Then the editor component opens in the wrong location on the first attempt, usually covering the row. See attached screenshot for what happens when opening the top row of a grid with editor components set.

Edit: Subsequent attempts seem to open correctly, for some reason it's only the first attempt that opens in the wrong location.
For a long time I thought this was an issue with vaadin itself. This is the problematic statement:
.v-grid-tablewrapper {
overflow: visible;
position: unset;
width: 100% !important;
}
You can fix it by adding this to your theme:
.v-grid-tablewrapper {
position: absolute;
}
.v-grid-scroller {
z-index: 6;
}