Steps to reproduce:

• Run this in the address bar:

data:text/html,<style>@supports (display: block) { @media (min-width: 1px) { body { color: green; } } }

• Open DevTools, switch to Inspector
• in the Rules panel, toggle off the color: green declaration
• in the Changes panel, right click on the @supports rule and pick "Copy Rule" from the context menu
• paste the clipboard content into a text document

Expected:

@supports (display: block) {
 @media (min-width: 1px) {
  body {
   /* color: green */
  }
 }
}

Actual result:

@supports (display: block) {
 @media (min-width: 1px) {
  body {
   color: green 
  }
 }
}

This patch adds an boolean option to skip the cached value of StyeRuleActor.auhtoredText and re-parse the stylesheet.
When changing content of a descendant rule inside an ancestor like @media or @select, the cached value is outdated.
This yields incorrect data when requesting the complete rule authored text, hence the need for a cache-busting variable.

In addtion to this change, there is a slight refactor to include the generated unique selector for the mock-rule for element inline styles.

https://hg.mozilla.org/mozilla-central/rev/ccfe228bff77