Develop alongside index.htm

When setting up a new site in the past, I used to post an index.htm file in the root directory with a “coming soon” notice, then develop the WP site in the same directory, accessing files by appending index.php to the URLs.

With one of the recent WP releases, this stopped working. All URLs redirect to the root, stripping index.php from the URL.

Where is this redirection happening? Is there something I can edit to get that staging capability back?