Moving exclusive fullscreen windows between monitors seamlessly

[theagentd]

Hello!

I've been exploring fullscreen windows lately, and I have noticed that there are some odd interactions when moving fullscreen windows between displays using Windows-key+shift+left/right arrow. This works completely seamlessly for borderless fullscreen windows, absolutely great there!

However, for exclusive fullscreen windows, it seems like it tries to bring over the set display mode to the new window, causing a display mode change to happen. It seems like when a window is moved to a different display using this shortcut, all displays will change to the closest display mode of the one initially set using SDL_SetWindowFullscreenMode() when the window ends up on them. This changed display mode remains as long as the program is running.

Example: I have three monitors:

• A 1440p 240Hz monitor.
• A 1440p 165Hz monitor.
• A 4K 120Hz monitor.

Start on the 240 Hz monitor --> 165Hz monitor stays the same; the 4K monitor switches to 1440p.
Start on the 165 Hz monitor --> 240Hz monitor switches to 165Hz; the 4K monitor switches to 1440p.
Start on the 4K monitor --> seamless switching, as the other ones can't do 4K.

Ideally, I would like to use the desktop display mode for all windows, just like for borderless fullscreen. To accomplish that, I attempted to detect the user moving a window from one display to another using this shortcut by looking out for SDL_EVENT_WINDOW_DISPLAY_CHANGED events, and then changing the display mode based on that. Unfortunately, this detection isn't fast enough, so my monitors still switch display modes before I can switch them back, resulting in a lot of flickering, 1-second freezes, etc.

Is there a way to get the same seamless movement between displays as for borderless fullscreen right now? Or is this something that would require changes in SDL to accomplish? Alternatively, can I disable the Win+shift+left/right shortcut for my SDL windows?

Thanks in advance!

[Kontrabant]

If real mode switches are taking place, no, it's not going to be smooth, as the display the window was on has to switch back to the desktop mode, and the new display tries to switch to the closest mode available. The only way to make it work seamlessly is to use desktop fullscreen and handle scaling yourself, or emulate mode switching with scaling on the GPU (I think the Nvidia drivers on Windows have an option for this, and the Wayland backend on Linux works this way as well).

Ideally, I would like to use the desktop display mode for all windows, just like for borderless fullscreen.

If you always want the desktop mode, you can just use fullscreen desktop, or do fullscreen desktop windows have some issue that make them unsuitable for your use case?

[theagentd]

As far as I'm aware, borderless fullscreen / fullscreen desktop can have slightly worse performance than exclusive fullscreen, so I would like to ideally provide it as an option to players. As I mentioned, borderless fullscreen is already working exactly how I want it to work.

The problem I want to bring up is that moving an exclusive fullscreen window from one display to another right now will cause the display mode to be transfered from the display it was intended for to one that has a completely different set of valid display modes, and there's no way for me to correct this before an automatic display mode switch takes place using event callbacks. Since SDL display modes specify a specific resolution/refreshrate AND a specific display, it seems unintuitive that it would try to apply the same resolution/refresh rate to a different completely display when a window is moved.

I hope that makes sense.

[Kontrabant]

Since SDL display modes specify a specific resolution/refreshrate AND a specific display, it seems unintuitive that it would try to apply the same resolution/refresh rate to a different completely display when a window is moved.

Many games, especially older ones, don't expect that an exclusive fullscreen window can move and be suddenly resized, so setting the new display to a matching mode is an attempt to keep them from breaking by making the move as transparent as possible. A hint could be added to disable this behavior for cases like yours, where you know this is possible and want to deal with it directly instead of having SDL do it.

[Kontrabant]

Upstream for 3.4.0, you can set the SDL_VIDEO_MATCH_EXCLUSIVE_MODE_ON_MOVE hint to false to handle any mode switching when windows move yourself.

[sezero]

Upstream for 3.10,

Meant 3.2.10, yes? Assuming yes, ...

you can set the SDL_VIDEO_MATCH_EXCLUSIVE_MODE_ON_MOVE hint to false to handle any mode switching when windows move yourself.

... #12540 isn't cherry-piched to release-3.2.x (yet)

[Kontrabant]

Thinking about it a bit more, this is a feature and slight behavior change as opposed to a bug fix, so it should probably wait for 3.4. I'll update the hint accordingly.

[theagentd]

Thanks for the really quick response! Much appreciated!

Looking at the pull request, I noticed this:

"0": SDL will not attempt to automatically set a matching mode on the destination display. If an exclusive fullscreen window is moved to a new display, the window will become fullscreen desktop.

I assume that this means that the correct usage of this hint is to detect the window change and then set a proper exclusive fullscreen mode when a SDL_EVENT_WINDOW_DISPLAY_CHANGED event happens? I have to say, that seems like a very clever way of handling it!