I bisected the regression to this SDL3 commit:

commit 5594d03da086ab255b1d7ace1496f3a0c109a83d (HEAD)
Author: Sam Lantinga <slouken>
Date:   Tue Sep 2 20:11:00 2025 -0700

    Leave letterbox borders set to the frame clear color
    
    Fixes https://github.com/libsdl-org/sdl2-compat/issues/483
    
    (cherry picked from commit fbbc29159a25f26d789694a74c2d1d3216dfb49d)


The Perl reproducer is:

$ cat t/test.t 
#!/usr/bin/perl -w
use SDL;
use SDL::Video;
use Test::More tests => 1;
$ENV{SDL_VIDEODRIVER} = 'dummy';
my $display = SDL::Video::set_video_mode( 640, 480, 232, SDL_ANYFORMAT );
isnt( SDL::get_error(), '', '[get_error] got error ' . SDL::get_error() );

$ LD_LIBRARY_PATH=/tmp/SDL/redhat-linux-build perl -Iblib/{lib,arch} t/test.t 
1..1
not ok 1 - [get_error] got error 
#   Failed test '[get_error] got error '
#   at t/test.t line 7.
#          got: ''
#     expected: anything else
# Looks like you failed 1 test of 1.

It has been a while since I've looked at perl-SDL, but it looks like it it's using the SDL 1.2 compatibility layer and not SDL2 or SDL3, which is fine, but makes this ... trickier for me to reproduce in C code?

I tried this reproducer case:

*****

#include <SDL.h>

int main(int argc, char* argv[]) {
   SDL_Surface* screen;

   SDL_Init(SDL_INIT_EVERYTHING);

   // This should work
   screen = SDL_SetVideoMode(640, 480, 32, SDL_SWSURFACE);

   // This should fail
   // screen = SDL_SetVideoMode(640, 480, 232, SDL_SWSURFACE);

   if (!screen) {
        printf("the mode could not be set\n");
        SDL_Quit();
        return 1;
   }

   // Flip the screen to make the surface visible (essential for SDL 1.2)
   SDL_Flip(screen);

   // Wait for 2 seconds before quitting
   SDL_Delay(2000);
   
   // Clean up SDL resources and exit
   SDL_Quit();
   return 0;
}

*****

I _think_ that's doing the same thing roughly as the perl-SDL testcode, but it fails when it is supposed to (comment out the first case, uncomment the second). A bit stumped.

And of course, when I type it in, I see my mistake. :P I'm using SDL_SWSURFACE, not SDL_ANYFORMAT. Switching to that and I can reproduce.

Okay, now I'm back to being confused (a fairly normal state for me). I downgraded to SDL3 3.2.20 to test things, but I get the same behavior there as I do on 3.2.24:

SDL_SetVideoMode(640, 480, 32, SDL_ANYFORMAT); <= works
SDL_SetVideoMode(640, 480, 232, SDL_ANYFORMAT); <= works
SDL_SetVideoMode(640, 480, 32, SDL_SWSURFACE); <= works
SDL_SetVideoMode(640, 480, 232, SDL_SWSURFACE); <= fails

Created attachment 2108855 [details]
test code with debugging for all four cases

gcc -I/usr/include/SDL -o SDL-example SDL-example.c -lSDL-1.2

For me SDL_SetVideoMode() passes with both SDL3 versions, but SDL_GetError() stopped reporting the error. My reproducer:

#include <SDL.h>

int main(int argc, char* argv[]) {
    SDL_Surface *screen;
    char *error;

    if (SDL_Init(SDL_INIT_VIDEO)) {
       printf("SDL_Init() failed\n");
       return 1;
    }

    screen = SDL_SetVideoMode(640, 480, 232, SDL_ANYFORMAT);
    if (!screen) {
        printf("SDL_SetVideoMode() failed\n");
        SDL_Quit();
        return 1;
    }

    error = SDL_GetError();
    if (error)
       printf("Last error = %s\n", error);
    else
       printf("SDL_GetError() did not return any error\n");

    SDL_Quit();
    return 0;
}

Old behavior:


$ SDL_VIDEODRIVER=dummy ./a.out; echo $?
Last error = rect has a negative size
0

New behavior:

$ SDL_VIDEODRIVER=dummy ./a.out; echo $?
Last error = 
0

The new behavior is at least consistent. But I worry that it actually returns a buggy SDL_Surface. Inspecting its details shows, in both versions:

w=640 h=480 bpp=32

I don't know whether SDL_SetVideoMode() always (since SDL 1.2) succeeded and only signalled an error with SDL_GetError().
It's quite possible that the perl-SDL test wanted to test SDL::get_error(), and SDL::Video::set_video_mode(640, 480, 232, SDL_ANYFORMAT) was only a tool for it. Then I could change the test to use call SDL::Video::set_video_mode(-1, ...).

(In reply to Petr Pisar from comment #6)
> It's quite possible that the perl-SDL test wanted to test SDL::get_error(),
> and SDL::Video::set_video_mode(640, 480, 232, SDL_ANYFORMAT) was only a tool
> for it. Then I could change the test to use call
> SDL::Video::set_video_mode(-1, ...).

I could not find anything helpful in the perl-SDL history. Then judging from the current test, the test is based on wrong assumptions and I will correct it.