So I had tried playing Thief a few years ago.
Never found a solution to strange video errors I was getting.

I just tried again with the New setup files.
Looks better from what I can remember, no purple and white pixels everywhere.

But can I improve the resolution anymore?

Game starts at 800x600
So the opening cinematic still looks kind of hazy.
Switching to 1920x1080 doesn't help, it makes the game screen smaller with black bars on either side.

I play both Thief 1 and Thief 2 in 1900x1200x32 and no black bars. The videos and/or opening cinimatic might look fuzzy (didn't pay attention to that) because it's an old game and perhaps they weren't recorded that high.

But the games themselves, running my GOG versions with whatever the NewDark they are on (I think I updated the T2 NewDark but don't recall for sure or when), runs great for me at my full monitor resolution, without black bars.

It's the menus that are cropped.
The videos do look slightly better in 1920x1080.
Actual gameplay is fine too. It's just the menu screens are smaller

Okay, gotcha.

Yep, my menus also have black bars on both sides (not the top or bottom). They're otherwise normal full-sized menus and text. But they do have black bars on the two sides (as if they weren't in 'widescreen'. But they're not shrunken down or anything, so it isn't that noticeable.

I tried to take a screenshot to show you; but apparently I don't have a utility that can grab the Thief menu screens (the Thief built-in key only works in-game, not in the menus).

The menus are not cropped and they're not widescreen.
The menus have always been designed for 4:3 monitors.
Newer versions of the engine can upscale them hence why they don't look like a mess of big pixels and are not stretched. I believe there is a way to force them stretching if you really want to (after all, to each his own ^^') though I don't remember what. I guess voodoo47 would know, and also seem to remember a thread about it on TTLG, but I don't have enough time to look for it right now (about to leave for work).

it's all in the docs.

You are right, they do not look like a big mess.

I'm satisfied, knowing that after switching it to the correct resolution, what I am seeing is the norm.
Really happy though that the video quality is improved!