TS2LC: Stuck on borderless windowed, can't get fullscreen or change resolution

Thanks, does this also happen on a brand new save file? 

You can move the saves to your desktop from Documents > EA Games > Sims 2 Legacy > Neighborhoods to test this. 

I wiped the entire game and started with an umodded, fresh install. Still have the same issues.

Thanks SimsMegaDeluxe​ 

For clarity, by wiped you mean you uninstalled the game, removed/moved the save folder, etc? The first time you launched the game after you had no family created?

If so could you please provide a DxDiag I can share with the team?

Yes, I uninstalled the game via Steam app (removing both the installation directory and save data in documents). I reinstalled the game and launched it immediately with no existing save file or mods (vanilla). Issues remained:

1. I am able to press the windows key and bring up the taskbar/open other application on top of the Sims 2, but switching applications causes the game's audio to mute. Thankfully, the game still runs and does not freeze. (The in-game option to "Sim while minimized" is checked ON)
2. In-game window resolution options still grayed out
3. Alt-enter does not work and causes Windows error chime to play

FWIW, the game's resolution looks nice and the UI scaled appropriately for my monitor.

Here is the DxDiag. Thank you for your attentiveness.

Thanks for all the information so far!

I've discussed this with the team and they had a few more suggestions they would like you to try:

What happens if you change this to other resolutions than 3840 x 2160? 

What happens if you open the GraphicsRules.sgr file, find maxResWidth and set it to 3500, then find maxResHeight and set it to 1900 and start the game? 

Also, If you start the game with -w added to the “Options” field in Steam, do you get a window with any decoration (titlebar, etc)?

What happens if you change this to other resolutions than 3840 x 2160? 

I assume you mean my Windows Display Settings because I can't change resolution within the game. I changed my Display Settings to a lower resolution, causing pillarboxing (vertical black boxes on left and right of my display). I ran the game and nothing changed, besides the pillarboxing on the two sides of my monitor. Still fullscreen windowed, alt-enter not working, and grayed out in-game settings.

What happens if you open the GraphicsRules.sgr file, find maxResWidth and set it to 3500, then find maxResHeight and set it to 1900 and start the game? 

I went to C:\Program Files (x86)\Steam\steamapps\common\The Sims 2 Legacy Collection\EP9\TSData\Res\Config and found "Graphics Rules.sgr" The screenshot at the bottom shows the default values all set at 16000 for maxResWidth and maxResHeight. I changed the pair of values for each of the four modes to 3500 and 1900. Saved the file and launched the game. No change.

Also, If you start the game with -w added to the “Options” field in Steam, do you get a window with any decoration (titlebar, etc)?

No change. No decoration as usual.

Thank you so much, I'll be in touch as soon as I hear back from the team. 

I am also having this issue and have the same settings as you. I'm very much considering refunding as I only just bought the game.

I hope you hear back from the team