Firmware version: v2.3.4

Using the following video modes, when the Dreamcast is producing 31KHz 480p output:

DC_VIDEO_CONFIG_1080P
DC_VIDEO_CONFIG_960P
DC_VIDEO_CONFIG_VGA

the visible display area is cut off by four pixels on the left-hand side, for the majority of commercial Dreamcast software.

There are some rare exceptions, like MSR, which is instead comparatively shifted eight pixels to the right (although MSR has its own built in display position config, so it is not a big problem).

Some homebrew software, such as GDmenu, is comparatively shifted four pixels to the right, and appears centred correctly.

The same discrepancies between software can be seen when using the OSSC configured for 640x480 video sampling, with the Dreamcast in 31KHz 480p output.

This area is not missing in 480p HDMI mode, when the Dreamcast is set to 31KHz 480p output:

DC_VIDEO_CONFIG_480P

I think this is because the full 720 horizonal pixels are displayed (although it is still possible the image is not centred correctly).

An easy test for this is to run Rez, which during gameplay frequently writes text partly across the display area that gets cut off on the left-hand side (in Rez the HUD elements are drawn closer to the left and right edges of the screen when the game is in 31KHz 480p output).

This area is also not cut off when the Dreamcast output is 240p, 480i, or 576i, with these video modes:

DC_VIDEO_CONFIG_576P
DC_VIDEO_CONFIG_240P_1080P
DC_VIDEO_CONFIG_240P_960P
DC_VIDEO_CONFIG_240P_480P
DC_VIDEO_CONFIG_240P_VGA
DC_VIDEO_CONFIG_480I
DC_VIDEO_CONFIG_576I

I'm happy to help further with this, thanks for taking a look.