The number to pay attention to on those screenshots: Video output has decreased from 4 ms to 2 ms with the SDL 2 driver.
Not tested it myself.

Also worth mentioning is that this changes the SDL driver from software rendering to hardware rendering, which has to potential to cause problems since we're now relying on the user's graphics drivers.

I've seen this branch segfault when the program starts on an old Mac mini I have, running Linux and the nouveau DRI drivers. This computer can run OpenTTD with SDL 1.2 just fine, and obviously we don't want to introduce this regression.

So I'll need to implement a software rendering fallback using either SDL_CreateSoftwareRenderer() or SDL_CreateRenderer() with the SDL_RENDERER_SOFTWARE flag. I'm not sure exactly how to "fall back" to the software renderer, but I'm sure there's some examples around. I'll take a look.

Alright, I've gotten a chance to debug the issue on the Mac mini system I mentioned above. It's working fine with an update I just made to this branch: using a renderer created with the SDL_RENDERER_SOFTWARE flag instead of SDL_RENDERER_ACCELERATED. Unfortunately, just passing in 0 here and letting SDL choose doesn't fix the problem: seems to be a driver bug, which other people have run into with these nouveau drivers.

Anyways, we're not losing anything by using software rendering here, as that's how it currently works. Please test this branch, anyone who's able and willing, and let me know of any problems you see!

Of course CI fails for linux because SDL2-dev is not installed

Hmm, I can't see the exact libsdl failure - though I'm not used to Azure.

I see a bunch of messages like "Unable to find image 'openttd/compile-farm-ci:linux-amd64-gcc-6' locally" here: https://github.com/OpenTTD/OpenTTD/pull/7086/checks?check_run_id=55198664

Anyways, this Dockerfile would need to be updated to include libsdl2-dev. I'll make a PR.
https://github.com/OpenTTD/CompileFarm/blob/master/base-linux/Dockerfile#L29

It fails on configure, https://github.com/OpenTTD/CompileFarm/blob/master/base-linux/Dockerfile needs an udpate

If you go to the Azure results, here: https://dev.azure.com/openttd/OpenTTD/_build/results?buildId=607
You can click a build that failed. Then you can click again on the step that failed. There you see: 2019-01-26T23:49:22.1701030Z configure: error: no video driver development files found. We have an open bug to make the reporting more verbose.

I have the same problem as @Nik-mmzd in #7296.

At the first start after compilation the NewGRF scanning window appears and the progress bar goes up to 95% and before it closes the screen freezes. The game itself keeps running because I can hear the sounds from the title game. Just nothing happens on the screen.

After I close the game, at the following game starts the screen freezes immediately. The NewGRF scanning window appears, but the progress bar stops at 0%. The game still running in these cases as well.

My system is a Fedora 29 on a physical hardware.

Software versions:
OpenTTD: 9124b35 from your sdl2 branch
Kernel: 4.20.15-200.fc29.x86_64
SDL2: 2.0.9 (both of 32 and 64 bit versions)
NVIDIA Driver Version: 418.43

openttd -d driver=1:
dbg: [driver] Successfully loaded blitter '32bpp-anim'
dbg: [driver] SDL: using mode 1920x1054x32
dbg: [driver] SDL: using driver 'x11'
dbg: [driver] Successfully probed video driver 'sdl'
dbg: [driver] Successfully probed sound driver 'sdl'
dbg: [driver] Successfully probed music driver 'extmidi'
dbg: [driver] Successfully loaded blitter '32bpp-sse2-anim'
dbg: [driver] SDL: using mode 1920x1054x32
dbg: [driver] SDL: using threads
dbg: [driver] SDL: using mode 1920x1054x32
dbg: [driver] extmidi: set volume not implemented

@stormcone Good to hear it works with the Nouveau driver, but let me know when you're able to test this with the Nvidia proprietary driver again.

@nikolas Thank you. :)

I tried out your new commit with the disabled hardware acceleration and now the game starts normally. I can use the menu and start a new game. So looks like that commit solves the problem. :)

Edit: I mean with the NVIDIA driver.

Confirmed, now runs

Simulation rate is still consistently running at 1.02x (for me) but that's the same as SDL1, so I'm not too concerned about it

I'm on nikolas/OpenTTD@8306506; linux; sdl 2.0.9
Found two bugs:

1. Numpad Enter does not acts like Enter
2. Arrow buttons do not move cursor but ctrl-right/left arrow moves cursor by a word

Also, thanks for fixed #7296

Wow, it works, thanks

@nielsmh this is ready for another review, since known issues are now fixed. Just fyi.