Placeholder iframe not shown on particular site using Firefox

[xofe]

Prerequisites

• I performed a cursory search of the issue tracker to avoid opening a duplicate issue
  • Your issue may already be reported.
• This is not a support issue or a question
  • Support issues and questions are handled at /r/uMatrix
• I tried to reproduce the issue when...
  • uMatrix is the only extension
  • uMatrix with default lists/settings
  • using a new, unmodified browser profile
• I am running the latest version of uMatrix
• I checked the documentation to understand that the issue I report is not a normal behavior

Description

A specific URL where the issue occurs

http://lyrics.wikia.com/wiki/Florence_%2B_The_Machine:Hunger

Steps to Reproduce

1. Open Firefox 60 or later (using a clean profile) and install uMatrix 1.1.20 or later.
2. Add the rule wikia.com nocookie.net * allow to the 'My rules' section in the dashboard. (Without this rule, the site will appear broken)
3. Visit the URL above.
4. Notice that the usual iframe placeholder document for the YouTube video embed at the top right of the grey lyrics box (an iframe from www.youtube-nocookie.com, which is blocked) is not shown.

Supporting evidence

This only affects Firefox 60 or later when used with uMatrix 1.1.20 or later. The issue isn't present when using Firefox 59 or older with the current version of uMatrix, or when using uMatrix 1.1.18 or older with the current version of Firefox. Because of this, I'm not certain whether this is a Firefox or uMatrix issue.

For Firefox, moz-regression gives a regression range that points to Bug 1193394.

For uMatrix, git bisect says gorhill/uMatrix@73c8da0 is the culprit.

Screenshot of last good revision (gorhill/uMatrix@8615f3b) on Firefox 60.0.2 :

Screenshot of first bad revision (gorhill/uMatrix@73c8da0) on Firefox 60.0.2 :

Your environment

• uMatrix version: 1.3.10
• Browser Name and version: Firefox 60.0.2

[uBlock-user]

I can reproduce on Nightly.

[uBlock-user]

Make sure you have disabled Collapse placeholder of blocked elements and Collapse placeholder of blacklisted elements from the Settings menu.

[gorhill]

I can reproduce. Appears to be a timing issue, when I stepped into the javascript code, the placeholder appeared properly.

[xofe]

Confirmed fixed. Thanks!