ZDoom

Seems like it.

Does -glversion 3 block renderbuffers? The feature relies on that being available.

Actually, on Linux, the scaling mode changes are only effective instantly in the OpenGL renderer. In Software I have to force a video reset (e.g.: by changing the resolution) for it to take effect. (I think I already reported this before)

I CAN confirm however that with -glversion 3 the resolution stays at native fullscreen no matter what is chosen.

It is not supported in the hardware renderer? Always thought it was since the Mac target already does it for its full-screen mode. Need to test this when I get home.

The scaling mode only has an effect when using the software renderer. This feature is not implemented in the OpenGL hardware renderer. The fullecreen issue, I believe is an SDL issue but that needs to be confirmed by a Linux developer.

Using latest stable GZDoom available from debian.drdteam.org on Ubuntu 17.10
(GZDoom g3.3.0 - 2018-03-25 12:41:39 +0200 - SDL version | Compiled on Mar 25 2018)

Changing the resolution in fullscreen, or changing the scaling mode, has no effect on the actual renderer: it is always rendered at the maximum size possible at whatever aspect ratio is chosen.

Screen resolution: 1680x1050 (16:10 ratio)

https://i.imgur.com/qloCkhk.png
Render at 1440x900, 16:10 ratio

https://i.imgur.com/FY7nBah.png
Render at 640x400, 16:10 ratio

https://i.imgur.com/H5jgWBu.png
Render at native resolution (1680x1050, 16:10 ratio) with Resolution Scale set to 320x200

Due to my GPU and OS combination, I'm forced to run with -glversion 3, so if this is a gl4-only feature, I feel like it should be made more clear or at least hidden like the VR options are.