How to fix Windowed mode at a specific resolution regardless of window resizing?

I've been playing FFXIV for about a year, and the way windowed mode has always worked for me until now is that I assign it my desired resolution, and then FFXIV keeps that specific resolution no matter what size and shape I adjust the window into. If I shrink my window, my UI elements are packed closer together and I lose some field of view if the window's size isn't 16:9, but the actual resolution stays the same and all my UI elements look normal.

However, when I logged in today, my resolution was all out of whack and I was getting 5 fps. Rebooting helped the fps but not my resolution. In addition, FFXIV is now dynamically adjusting its resolution whenever I change FFXIV's window size and screwing up the relative sizes of my UI elements and giving the message, "Screen resolution does not meet current setting requirements. UI scaling has been readjusted."

For context, my computer's recommended resolution is 2560/1440 + 150% app scaling, and I've been using 1707/960 as FFXIV's resolution up until now. I checked windows and there hadn't been any windows or driver updates since yesterday.