Idea Summary
This was originally reported to Oracle Support as a bug, but they said it is not a bug and should be submitted as an enhancement request here.
When using a Region Display Selector on a page, if a user clicks on one of the areas that would show the proper region in the region display selector before the page has finished loading, they will be taken to the APEX Developer Sign-In window.
Can be seen here: https://apex.oracle.com/pls/apex/r/adptest/bug-demo/tab-region-flicker
If you refresh the page and quickly click on Fields or IR1 before the page has finished loading, you will be taken to https://apex.oracle.com/pls/apex/
Use Case
Some users have slower PCs or slower network connections than others. If there are pages that have multiple regions or lazy loading enabled, by clicking on one of the selections in the region display selector before the page has finished loading, it would confuse the users (at the APEX Developer Login page) causing bug reports and a complete break in user experience.
Preferred Solution (Optional)
My suggestion is to disable the selections from a Region Display Selector until the page has finished loading to prevent users from navigating somewhere unintended.