Mobile Device Issue Only : Closing Modal Dialog, hiding upper portion of content behind fixed header

### Issue type

**I'm submitting a ...**

* [x ] bug report

Mobile Device Issue Only : Closing Modal Dialog, hiding upper portion of content behind fixed header.
**High priority issue, as whole site navigation hides upper portion of content behind fixed header once modal dialog close.**

**Current behavior:**
Scroll down some content, open dialog and Closing modal dialog, and when we scroll up, we find hiding some upper portion of main content behind fixed header menu. Navigate to any page, and we will find the same issue at every page.

**Expected behavior:**
Closing modal dialog, and when we scroll up, should not hide main content behind header menu.

**Steps to reproduce:**
1. Open this link "https://www.akveo.com/ngx-admin/pages/modal-overlays/dialog" in mobile device.
2. Scroll down some content and open dialog.
3. Close Modal Dialog
4. Scroll up at top again
5. Some portion of the main content hides behind header menu.

### Other information:
Node Version: 12.16.1 and npm Version: 6.14.4
OS: Windows (10).
Browser: Chrome/Edge - Issue Reproducible
**Firefox - Issue Not Reproducible**