Disable scrolling while "Learn more" popups are active

[softvision-oana-arbuzov]

URL: https://webcompat.com/issues/new, https://staging.webcompat.com/issues/new

Browser / Version: Firefox Preview Nightly 200610 (🦎 79.0a1-20200609092134), Chrome 83.0.4103.101, Firefox Nightly 79.0a1 (2020-06-10) Operating System: Samsung Galaxy S6 Edge (Android 7.0) - 1440 x 2560 pixels (~577 ppi pixel density), Huawei P20 Lite (Android 8.0.0) - 1080 x 2280 pixels, 19:9 ratio (~432 ppi density), Windows 10 Pro

Steps to Reproduce:

1. Navigate to https://webcompat.com/issues/new
2. Tap "Learn more" link.
3. While popup is active, scroll up/down and observe behavior.

Expected Behavior: Page remains fixed in the background.

Actual Behavior: Page scrolls up/down in the background.

Notes:

1. Gif attached.

Watchers: @softvision-oana-arbuzov @cipriansv

[soniasingla]

@miketaylr can i take this bug? (:

[miketaylr]

@miketaylr can i take this bug? (:

Please and thank you!