In bug 1478397, we will land the very first version of the flex item sizing sidebar. For now, it will contain a flat list of properties that relate to where a flex item got its size. Now, in this second bug, we will transform this list into something that contains more information, and more user-friendly information. The goal being to explain in a straightforward way how an item really got its size.

WIP patch just to show the progress made so far. I need to wait for the blocker bug to land first and then I'll rebase this. I still need to polish this patch up and write tests.

Still a lot of work to do here, but I'd like to try to land this in 64 anyway. It's not a big deal because we plan on keeping the feature hidden behind a flag for now. The reason is that there is some actor code that landed in 64 with bug 1478397, and I'm changing it in this bug. So I'd rather have those changes in 64 too and avoid messy backwards compat code. So I'll polish up this patch and fix the obvious issues, but won't wait until it's 100% robust before landing. We'll do this through follow-upo bugs and will only turn the flag ON when things are stable anyway.

MozReview-Commit-ID: 3cSeShKP6TE

MozReview-Commit-ID: 3cSeShKP6TE

Pushed by pbrosset@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/6f5483bd51ed More user-friendly Flex item sizing information; r=gl