Support negative monitor indices in workspace rules for dynamic multi-monitor setups

[cmihail]

Describe the feature

Add support for negative monitor indices in workspace rules to enable dynamic multi-monitor configurations that adapt based on the number of connected monitors.

Currently, workspace rules use absolute monitor IDs:

workspace = 1, monitor:0, persistent:true, default:true
workspace = 2, monitor:1, persistent:true, default:true

This proposal would allow negative indices (counting from the end):

workspace = 1, monitor:-2, persistent:true, default:true
workspace = 2, monitor:-1, persistent:true, default:true

Behavior:

• Negative indices count backwards from the last monitor (like Python's negative indexing)
• -1 = last monitor, -2 = second-to-last monitor, etc.
• If the negative index exceeds available monitors, fallback to monitor:0

Examples:

With 3 monitors (IDs: 0, 1, 2):

• monitor:-1 → monitor 2
• monitor:-2 → monitor 1
• monitor:-3 → monitor 0

With 2 monitors (IDs: 0, 1):

• monitor:-1 → monitor 1
• monitor:-2 → monitor 0
• monitor:-3 → monitor 0 (fallback)

With 1 monitor (ID: 0):

• monitor:-1 → monitor 0
• monitor:-2 → monitor 0 (fallback)

Why would it be beneficial

This feature would solve the common use case where users have varying numbers of monitors in different setups (e.g., laptop with/without external displays, different docking stations).

Current problem:
Users must either:

1. Maintain separate configs for different monitor setups
2. Use scripts to dynamically generate configs on monitor connect/disconnect
3. Manually reconfigure workspace assignments when changing setups

With this feature:
A single static config would work across all setups:

# Rightmost monitor always gets workspace 2
workspace = 2, monitor:-1, persistent:true, default:true
# Second-from-right always gets workspace 1
workspace = 1, monitor:-2, persistent:true, default:true

This creates predictable, consistent workspace placement regardless of how many monitors are connected, improving the user experience for mobile/hybrid workflows.

[cmihail]

I can try to implement this feature myself if it looks good.

[vaxerski]

I dont mind

[cmihail]

Unfortunately negative IDs won't do what I want because ID assignment is based on monitor connect order, which is not something predictable for my environment.

But as alternative we can use position which is reliable for setups like:

monitor=eDP-1,preferred,0x0,1 # left monitor
monitor=HDMI-A-1,preferred,1920x0,1 # middle monitor
monitor=DP-1,preferred,auto-right,1 # right monitor

where the position is always deterministic even if having 1, 2 or 3 monitors no matter the order the monitors are attached.

Here is the PR: #12452

[cmihail]

PR ready for review as got it working.