[Affected versions]: - Firefox 45.0, Build ID 20160303134406 - Nightly 48.0a1, Build ID 20160317030235 (without e10s activated) [Affected platforms]: - Windows 7, Windows 10 - Mac OS 10.11 - Linux 14.04 [Prerequisites]: - Disable e10s feature if the browser under test supports it. [Steps to reproduce]: 1. Open the browser and the attached testcase. 2. Spam click the "Add Content" button (I used an auto clicker software). 3. Observe the browser behavior while the content is added. [Expected results]: No matter how long the button is spammed, the browser remains stable and responsive. [Actual results]: Browser randomly hangs for short periods (not responding state) when spamming the "Add Content" button. [Notes]: - This behavior usually occurs when Firefox / Nightly process reaches values between 400MB and 900MB of RAM memory used. - I have tried to reproduce this with e10s enabled but after 2GB of RAM memory used, didn't encountered any hangs.

visibility: hidden elements still take up space, they just don't display, and descendants can be visibility: visible, so layout still needs to be performed.