As you can see from the screen capture below, despite having a z-index of 2147483646, the overlay still appears under an advert.
I can't believe the advert has a higher z-index.
Is this a programming thing? Something to do with how the overlay layer works?
Hey Stu, thought I answered this question for ya a few months ago. The reason is, as @HWS says, due to to there essentially being no 'standard' for z-index values. People can use whatever number they want (afaik up to 32-bit signed, 2^31 - 1, aka a really big number ) and so you never know what they are going to have.
If you bring this issue to Google's attention, the results wouldn't be good as they will say that your drop-downs should never show on top of ads, which to me isn't really fair but I didn't write their terms.