Native indicator renderer for Linux (X11)

[poodle64]

Overview

Add native rendering support for Linux to match the macOS implementation. Currently Linux uses WebView fallback.

Background

• macOS native renderer completed in Phase 1: Add native rendering dependencies and foundation #24, Phase 2: Implement all indicator styles with animations #25, Phase 3: Integrate with audio pipeline and replace WebView #26 (NSWindow + Core Graphics)
• Windows/Linux use WebView fallback (working but slower)
• Linux has X11 overlay capabilities that can match macOS performance

Technical Approach

X11 Implementation:

• Use x11rb crate (already a dependency for mouse tracking)
• Create borderless override-redirect window
• Use XRender extension for compositing/transparency
• Blit SoftwareRenderer pixmap via XPutImage or XRender

Key Considerations:

• X11 only (Wayland compositors don't allow arbitrary overlay positioning)
• Detect X11 vs Wayland at runtime
• Fall back to WebView on Wayland
• Reuse existing SoftwareRenderer from Phase 2

Implementation Pattern

Follow the macOS pattern from src-tauri/src/recording_indicator/native.rs:

#[cfg(target_os = "linux")]
mod linux {
    use super::*;
    use x11rb::connection::Connection;
    use x11rb::protocol::xproto::*;
    
    pub struct NativeIndicator {
        conn: /* X11 connection */,
        window: Window,
        renderer: SoftwareRenderer,
        // ...
    }
}

Files to Modify

• src-tauri/src/recording_indicator/native.rs: Add Linux implementation
• src-tauri/src/recording_indicator.rs: Update platform checks
• src-tauri/src/audio/preview.rs: Update platform checks
• src-tauri/Cargo.toml: Ensure x11rb available (already is)

Acceptance Criteria

• Native indicator works on X11-based Linux
• Graceful fallback to WebView on Wayland
• All three styles render correctly (pill, cursor-dot, fixed-float)
• <1% CPU usage during recording
• <50ms show latency

Performance Target

Match macOS metrics:

• Zero IPC overhead (direct function calls)
• <1% CPU usage
• <50ms show latency

Testing

• Test on X11 (Ubuntu 22.04, Fedora, Arch)
• Test Wayland fallback (Ubuntu Wayland session)
• Verify visual quality matches WebView
• Profile CPU/memory usage

Dependencies

• Requires Phase 3: Integrate with audio pipeline and replace WebView #26 (Phase 3) ✅ Complete
• Requires Linux testing environment

References

• macOS implementation: src-tauri/src/recording_indicator/native.rs
• x11rb examples: https://github.com/psychon/x11rb/tree/master/examples
• XRender extension: For compositing/transparency

Notes

• Windows implementation not planned (WebView works well enough)
• Wayland cannot support arbitrary overlay positioning (compositor restriction)
• Linux users on Wayland will use WebView fallback (expected limitation)