Broken Sword: The Sleeping Dragon FMV flicker

[Triticum0]

Title

https://xemu.app/titles/5451001d/#Broken-Sword-The-Sleeping-Dragon

Bug Description

This game has a flickering video intro unrelated to the games in #1049

ss.mp4

Expected Behavior

Should not flicker
https://youtu.be/USvDHDYVTdE

xemu Version

Version: 0.7.58
Branch: master
Commit: 6f878ed
Date: Wed Jul 6 02:14:58 UTC 2022

System Information

Field	Value
OS	Windows 10
CPU	AMD Ryzen 5 2600 Six-Core Processor
Graphics Device	NVIDIA GeForce RTX 3060 Ti/PCIe/SSE2
Graphics Driver	4.0.0 NVIDIA 512.95

Additional Context

No response

[abaire]

This appears to be the same underlying issue as #1183. As with that game, I see occasional CPU writes to the framebuffer with NV097_CLEAR_SURFACE afterwards.

Clear 3c4
CPU write to 3c4
Flip to 3c4

Clear 3db
Flip to 3db

Clear 3c4
Flip to 3c4

Clear 3db
Flip to 3db

Clear 3c4
Flip to 3c4

Clear 3db
Flip to 3db

Clear 3c4
Flip to 3c4

Clear 3db
CPU write 3db
Flip to 3db

It's unclear to me how this could work, unless some CPU writes are missing or there's some mechanism that prevents the clear from affecting the display buffer.

I attempted to reproduce via a trivial test ({clear surface, CPU blit, flip} in a loop) but xemu detects the CPU blits without any problem and I never see a bad frame.