The best overview of
z-index and stacking contexts that I've come across, plus it keeps getting better as Josh extends the useful tools section 👏👏
Top of the tips is
isolation: isolate, a very useful CSS function that I never remember; it effectively resets the stacking context, which means it's particularly useful for component-based UI structures.
Also a worthy shout-out to the CSS Stacking Context Inspector plugin (also available for Chrome). It's already helped me locate a rogue stacking context being created by a global
opacity attribute (of all things 😂).