BTRFS installation not creating @ subvolume structure

[axellieb]

Distribution:
NAME="Pop!_OS"
VERSION="22.04 LTS"
ID=pop
ID_LIKE="ubuntu debian"
PRETTY_NAME="Pop!_OS 22.04 LTS"
VERSION_ID="22.04"
HOME_URL="https://pop.system76.com"
SUPPORT_URL="https://support.system76.com"
BUG_REPORT_URL="https://github.com/pop-os/pop/issues"
PRIVACY_POLICY_URL="https://system76.com/privacy"
VERSION_CODENAME=jammy
UBUNTU_CODENAME=jammy
LOGO=distributor-logo-pop-os

Related Application and/or Package Version:
N/A - This is an installer issue

Issue/Bug Description:
When installing Pop!_OS with BTRFS as the root filesystem using custom partitioning, the installer does not automatically create the expected @ subvolume structure. This causes issues with Timeshift, which requires the @ subvolume to function properly. After installation, sudo btrfs subvolume list / shows no subvolumes created.

Steps to reproduce (if you know):
Start Pop!_OS installation
Choose custom partitioning
Select BTRFS as filesystem for root partition
Complete installation
After first boot, run sudo btrfs subvolume list / to verify absence of @ subvolume

Expected behavior:
The installer should automatically create an @ subvolume for root when BTRFS is selected as the filesystem, similar to other Linux distributions using BTRFS.

Other Notes:
This requires manual post-installation configuration to create the @ subvolume structure for Timeshift to work properly
The fix requires creating an @ subvolume, copying the entire root filesystem into it, modifying fstab, and updating boot parameters
This could be particularly challenging for users who want to use Timeshift for system snapshots