When a modal window is open - the document behind the modal should not be scrollable.

Needs investigation - setting overflow:hidden on the root document/window does not seem to work on iOS or android.

Google says something like this:
document.ontouchstart = document.ontouchmove = function(event)

But that would prevent all scrolling - even within the modal.

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Update following https://tracker.moodle.org/browse/MDL-39851?focusedCommentId=238431&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-238431:

These are more issues related to scrolling on Android 4.2 Chrome, everything must be fixed (split this issue in multiple issue if needed):

• the file picker is scrollable horizontally even thought it could fit.
• the file picker is scrollable horizontally due to content that are over the screen width(like the license field that is too big)
• the file picker is scrollable vertically way over the end of the file picker (you'll see the page or/and the select/upload dialogue from tinymce)
• if you scroll vertically too far, then you can't scroll back to the top (Nexus 4 - Android 4.2.2 only)
• when the virtual keyboard is displayed then the window is lift up, causing the bug to see the parent page. Don't forget to recentre the page on the field the user is typing in.