[BUG] Game will not stay in full-screen mode

I'm playing PS2 for the first time in about 2 months. For some reason now when I try to play the game will not play in full-screen mode; it keeps switching to the desktop after one or two seconds play, and I have to Alt+Tab my way back in.

I've been playing PS2 full-screen for years; I don't know why it is doing this now. I just updated my GeForce drivers, but the problem remains. Any suggestions? Thanks.

Windows 10 Home v2004

Intel Core i7-5820K Haswell-E 6-Core 3.30 GHz (no overclock)
ASUS X99-A Motherboard / Crystal Sound 2 audio
32GB (4 x 8GB) 288-Pin DDR4 SDRAM DDR4 2400 (PC4 19200)
850w Power Supply
ASUS GeForce GTX 1080 8GB ROG STRIX v452.06

Monitor: Dell UltraSharp 27 QHD U2715H
Resolution: 2560x1440 (monitor native)

Logitech G910 USB Wired Gaming Keyboard
Logitech G502 Wired Gaming Mouse
Logitech G633 Artemis Spectrum Gaming Headset
Razer Tartarus Gaming Keypad

Problem solved! I found and deleted UserOptions.ini (made a backup first) and now PS2 is working great in full-screen mode.

Thought I had this problem solved, but for some reason today it won't stay in full-screen mode again; it keeps dropping back to desktop. Any suggestions?

Okedoke, I think I've finally got this figured out. My monitor display was set to 2560x1140, but PS2 insists on running at 2048x1152. Changing my monitor to display to 2048x1152 before running PS2 fixed the problem, at least this time around.

Are you sure "2560x1140" not 2560x1440 ?
for 1440 try to edit useroptions.ini (whole section below):

[Display]
FullscreenRefresh=0
Maximized=0
RenderQuality=1.000000
FullscreenWidth=2560
FullscreenHeight=1440
WindowedWidth=2560
WindowedHeight=1392
Mode=Fullscreen
FullscreenMode=Fullscreen

A must:
WindowedWidth less or equal FullscreenWidth
WindowedHeight < FullscreenHeight

if it helps but the game returns to a windowed mode set useroptions.ini as a read-only

Thanks for the suggestion InexoraVC! "1140" was a typo; my native resolution is 2560x1440.

Before I read your post I was still having issues, this time with PS2 for reasons unknown insisting on 1680x1050 for full-screen, and it wouldn't let me choose any other resolution. I then went into useroptions.ini and manually input 2560x1440, and it worked, just like your suggestion. I'll implement your other suggestions and see what happens. Making useroptions.ini read-only is a great idea.

Thanks again!

Things are working great! Thought I would mention one minor detail: I have Vertical Sync enabled to stop screen tearing, but whenever I start a new session the Vertical Sync isn't working. The solution is to go into the graphics settings a toggle the Vertical Sync option, and then it works. Not a big deal, but I've noticed this problem since the beginning of this year. Thanks.

Nice
About VSync - check Nvidia (or AMD) global settings. They may conflict with the settings in your game. And check the settings in useroptions.ini:
[Rendering]
VSync=1
And yes it can be a PS2 bug

Thanks for the tip, InexoraVC. I reset my Nvidia 3D settings, but the screen-tearing issue remains. It's not a big deal, but something I need to attend to every session.