various things I pick up along the way.

Z-Index and Stacking Contexts

When you give an element a z-index, that value is only compared against other elements in the same context. By default, an HTML document only has one context. Here's an example of one way to create a context:

 .some-element {
   position: relative;
   z-index: 1;

Other ways to create stacking contexts:

More on this: What the heck, z-index? - Josh Comeau


Flexbox help in Chrome Dev Tools

click the gray box next to a flex property to inspect the different properties:

Recommended Flexbox Reading

A complete guide to Flexbox - CSS Tricks