We got an alert earlier today with increased SpeedIndex on beta.
Here:s what it looked like before:
See what it looks like to users in the videos: http://webpagereplay-wikimedia.s3-website-us-east-1.amazonaws.com/?prefix=beta/mobile/chrome/100/Diabetes_in_dogs/2019-10-03-22-33/video/
There's a big grey area, I can see it myself when I access https://en.m.wikipedia.beta.wmflabs.org/wiki/Barack_Obama multiple times.
Based on the synthetic test videos, it started happening between 2019-10-03-20-14 and 2019-10-03-22-31 GMT
The grey box is the animation of the main menu that flashes when some of the earlier CSS is loaded by the browser.
This commit looks like the culprit: https://gerrit.wikimedia.org/r/#/c/mediawiki/skins/MinervaNeue/+/534888/
It was merged exactly during that timeframe and shifts some code around related to the menu's animation: https://gerrit.wikimedia.org/r/#/c/mediawiki/skins/MinervaNeue/+/534888/35/resources/skins.minerva.mainMenu.styles/MainMenu.less
Making this a UBN because it should be a train blocker if it's not addressed quickly.