Building a Desktop Widget with Tauri v2

CostDog is a desktop widget that sits on top of your terminal. Here's how we built it with Tauri v2.

Why Tauri?

**3MB installer** vs Electron's 150MB+

**Rust backend** — fast, memory-safe, no runtime

**System WebView** — no bundled browser

**Native feel** — real window controls, system tray

The Architecture

┌─────────────────────┐

│ Tauri WebView │

│ (HTML/CSS/JS) │

├─────────────────────┤

│ Rust Backend │

│ (SQLite, Scanner) │

├─────────────────────┤

│ System Window │

│ (Always on top) │

└─────────────────────┘

Key Design Decisions

1. Frameless Window

We used `decorations: false` to remove the native title bar, then built our own draggable bar with a close button.

2. Dynamic Resizing

The widget starts at 410×36px (just the bar) and expands to 410×520px when clicked. We use Tauri's `resize_window` command for this.

3. Auto-Refresh

A background thread in Rust scans session logs every 30 seconds and emits a `refresh-data` event to the frontend via Tauri's event system.

4. Local SQLite

All data is stored in `~/.costdog/costdog.sqlite` using rusqlite with WAL journal mode for concurrent reads.

Challenges

1. **macOS dragging** — CSS `-webkit-app-region: drag` doesn't work reliably. We switched to Tauri's `data-tauri-drag-region` attribute.

2. **Gatekeeper** — Unsigned macOS apps are blocked. Users need to run `xattr -cr` to unblock.

3. **Window focus** — Always-on-top windows can steal focus. We had to handle click-through carefully.

Code Example

Here's how the Rust backend scans Claude Code sessions:

fn scan_claude_sessions(db: &Connection) -> Result<()> {

let home = dirs::home_dir().unwrap();

let claude_dir = home.join(".claude").join("projects");

for entry in WalkDir::new(&claude_dir) {

let entry = entry?;

if entry.path().extension().map_or(false, |e| e == "jsonl") {

parse_jsonl(entry.path(), db)?;

}

}

Ok(())

}

Lessons Learned

1. **Start simple.** We started with a single HTML file. No build step, no framework.

2. **Rust is worth it.** The learning curve is steep, but the performance and safety pay off.

3. **Desktop apps are different.** Window management, system tray, auto-update — all need special handling.