Win + Ctrl + Shift + B — resets the graphics driver and Desktop Window Manager without a full reboot. If this instantly fixes it, that's strong confirmation of software, not panel hardware.
Event Viewer → Windows Logs → System → filter for Event ID 4101- If entries cluster right around the times the screen goes half-black, that's your driver crashing and partially failing to repaint — not a hardware fault.
Download the Surface driver/firmware .msi directly from Microsoft's support page for your exact mode.
Connect with an external monitor- note whether the external display is affected.