Summary:

When opening a custom ASP page, the page does not load fully and when the page is resized, the vertical scroll bar is missing.

Resolution:
In one particular instance there was a malfunctioning custom content script on the screen the customer was navigating to.
This script was referencing a field that didn't exist on that screen. When this script was removed the issue was no longer reproducible.

In order to debug such issues the browser's console page can be used by selecting F12 in the browser. If you perform the failing actions again you may see some javascript errors in the console. It may not always be 100% obvious as to the exact cause however any javascript errors will point out the first area of code which is experiencing issues which can help you drill down through the problematic javascript to identify the cause.

To help prevent custom content issues, all custom content should be tested thoroughly in multiple browsers and browser versions to ensure that no issues have been introduced into the system.