Your system information

• Steam client version (build number or date): 0.20240610.0+srt1
• Distribution: Gentoo
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes
• Steam Logs: [generate by running this command in a terminal tar -zcvf ~/Desktop/steam-logs.tar.gz ~/.steam/steam/logs]
• GPU: Nvidia

Please describe your issue in as much detail as possible:

Launching steam in Big Picture Mode window disabled and I expect steam to display fullscreen. Steam is placed in the upper left corner but does not consume the entire display. If I switch to windowed and maximize steam then disable windowed the client will display Fullscreen (very laggy). On exiting and restarting steam the display size resets to the previous value and is not Fullscreen.

Steps for reproducing this issue:

1. Launch Steam with -bigpicture
2. Observe window is not fullscreen

config.vdf contains the following section where it appears to somehow use 1280x800 instead of 4096x2145. Modifying this file, does not keep the value and resets on startup.

 "UI"
        {
                "display"
                {
                        "Current"
                        {
                                "MinScaleFactor"                "0.5"
                                "MaxScaleFactor"                "1.49847817420959473"
                                "IsExternalDisplay"             "1"
                                "name"          "External: Q80A 11\"|||Fullscreen-1280x800"
                                "AutoScaleFactor"               "1"
                                "ScaleFactor"           "1"
                        }
                        "External: Q80A 11\"|||Fullscreen-4096x2145"
                        {
                        }
                        "External: Q80A 11\"|||Fullscreen-1280x800"
                        {
                        }
                }
                "Window"
                {
                        "SP"
                        {
                                "x"             "1408"
                                "y"             "680"
                                "width"         "1280"
                                "height"                "800"
                        }
                }
        }