STR:

1. visit https://codepen.io/rachelandrew/pen/axLzYv?editors=1100
2. enable the grid highlighter for the parent
3. enable the grid highlighter for the subgrid

AR:

• both highlighters are checked. the parent is translucent and covers the subgrid numbers

ER:

• when both highlighters are checked, both should be fully opaque with the subgrid numbers layering over the parent highlighters

In general, depending on the order of clicks the highlighter can end up in weird states. What I would expect is:

• if only the parent is checked, only show the parent highlighter
• if only the subgrid is checked, show the subgrid highlighter fully opaque with the translucent highlighter of the parent below
• if both are checked, show both highlighters fully opaque with the subgrid highlighter being on top of the parent

The order of activation should not matter.

This fixes 2 issues that we had when the subgrid and parent grid highlighter are shown:

1. When the subgrid and its parent grid container are both highlighted and the parent grid container
   is unhighlighted, we need to still show a translucent parent grid container highlight.

2. Avoid showing a translucent parent grid container highlight for a subgrid, when the
   parent grid container is already highlighted.

https://hg.mozilla.org/mozilla-central/rev/de2d79704486

Confirmed with 69.0a1 (2019-06-12).
Fix verified with 69.0b11 on Windows 10, macOS 10.13, Ubuntu 16.04.