We're encountering some strange compositing behaviour with the keyboard when a semi-transparent image is used for the key magnifier and the extended key popover. When they overlap the top of the keyboard their composited result is much darker than it should be (see composite_error_1.png for an example).
I believe this might be because Mir is compositing with the invisible layer that the shell places over applications to darken them when the dash is displayed via a swipe from the left. This would seem to account for the right amount of darkness added (see composite_error_2.png).
I also tested with a pure white background at 10% opacity, displayed over a white app (white-test.jpg), which shows the same effect, but makes it clear that compositing is happening as the "Activity" text can be read through the overlay; so it's not just being composited against a black surface or something, but seemingly with something else that is partially transparent and the overlay from the shell would seem the most likely candidate.
We're likely to work around this as much as possible in the keyboard by making the main popover background fully opaque for the time being, but this still causes issues for the drop shadow surrounding the popover.
