For the test, you need to create a second vidtual drive with BTRFS partition with enough disk space (~10-15 GB, although I tested it with 100 GB), which you need to connect then to your VM. I tried to download CS:GO on BTRFS, since it doesn't want to install on FAT32 partition, but ReactOS system partition must be FAT32.
It is not possible to reproduce this if to install ReactOS on BTRFS, since BSOD 0x3f appears when trying to extract Steam on drive C:\, so it can't be extracted fully.
- Download Steam from Rapps.
- Extract it to C:\Program Files.
- Download modified config.vdf here: https://archive.org/download/steam_201901/config.vdf and place it in C:\Program Files\steam\config with overwriting original version, in order to be able to download games.
- Create link onto C:\Program Files\steam\Steam.exe on Desktop with -no-cef-sandbox parameter, in order to be able to load web-pages in Steam.
- Create a folder "Steam" on the BTRFS partition. It will be a directory for Steam Library.
- Launch Steam.
- Login into your account.
- In Settings -> Downloading -> Steam Library Folders, add a folder mentioned above and make it default.
- Open Marketplace and find "Counter Strike: Global Offensive".
- Open its page from the list and click "Install". When the wizard will ask where to install the game, selesct drive with BTRFS partition and with "Steam" folder there, or another dir if you named it differently.
- The downloading will start. Wait some time. Sooner or later, the system necessarily will crash to BSOD 0x3f. At the very outside, it will crash at the end of downloading, when moving game files from "downloading" directory to "common".
This happens when I try to install the game only on BTRFS partition. On Ext2/3/4/NTFS with write support enabled, it doesn't happen, but the game can't install correctly anyway.
Tested with 0.4.13-dev-241-g63286c6, on VirtualBox 5.2.26 with Guest Additions and Mesa3D installed.