feat: focus ring fade animation and gradient rotation

[oblivion8282-1337]

Summary

• Fade animation: Focus ring smoothly fades in/out on focus changes using an EaseOutCubic curve, instead of appearing/disappearing instantly.
• Gradient rotation: The focus ring gradient continuously rotates while a window is focused, adding a subtle dynamic effect.

Both features are fully opt-in via the focus-ring {} config block:

focus-ring {
    width 2
    active-gradient from="#7aa2f7" to="#bb9af7" angle=45
    fade-duration-ms 1000
    gradient-spin-speed 90
}

• fade-duration-ms — duration of the fade in/out (0 or omitted = off)
• gradient-spin-speed — rotation speed in degrees/second (0 or omitted = off)

No behavior changes when these options are not set.

Changed files

• niri-config/src/appearance.rs — new config fields
• src/layout/focus_ring.rs — gradient angle offset parameter
• src/layout/tile.rs — fade animation + gradient rotation logic
• src/layout/insert_hint_element.rs — pass through new fields
• src/ui/mru.rs — pass through new fields

Test plan

• Tested fade with various durations (200ms–1500ms), 500–1000ms feels best
• Tested gradient rotation at different speeds (45–180 deg/s)
• Verified no effect when config options are omitted
• Verified no performance impact in normal use

🤖 Generated with Claude Code

[Anderson0xFF]

This is amazing! I was looking for something similar for Niri, since it currently doesn't support animated borders.

[Ozogorgor]

I haven't been using Niri for long (~3 weeks), but from what I can tell decoration rendering (borders, focus rings, etc.) currently isn't part of the shader pipeline.

If decoration rendering could be exposed to shaders, it might enable shader-based effects or animations for borders and focus rings. A shader implementation would give me more control over the type of animation I want of my decorations. I also might be completely wrong with my assumptions, but these are my two cents.