CSS locks scroll on Tryit Editor!
From https://www.w3schools.com/css/css3_backgrounds.asp to https://www.w3schools.com/css/tryit.asp?filename=trycss3_background_multiple
When a shorthand url(img_tree.gif) is added to the mix with attachment property fixed value, or when existing longhand adds url(img_tree.gif), then the mouse-wheel scroll locks, cannot scroll. Side scroll-bar can be dragged to scroll. Oddly, sometimes scroll-bar use will unlock, sometimes refresh page and paste 'locked' code will unlock. Some setting is 'teettering-on-the-edge'!
Tryit Editor bug. No files to submit. Reproduceable.
My mach is MacPro late 2016 with 10.12.4 and Apple wired keyboard and Apple Wireless Mouse same date using Safari 10.1 (12603.1.30.0.34). Error reproduced twice: shorthand/longhand mix and all longhand with bug condition being addition of third image.