I don’t think its for vertical centering since the child div has fixed width and height which expands the parent. Removing the styles in question(top, left, transform) from the element does not change anything regarding its position and dimensions, that’s why it’s confusing to me why they are applied in the first place.
To clarify about the background, in my test case I have a div with width and height of 100% with a solid background color. The CSS translate from the parent div.extension-overlays causes 1px from the top and left of my div to be semi transparent which is probably a rendering bug from Chrome (not sure). This is not a big issue to consider but to apply CSS transformation to a heavy DOM structure is pretty demanding task for the browser and maybe that’s why I see strange rendering issues.