> dreb can create TUI components. Ask it to build one for your use case.

# TUI Components

Extensions and custom tools can render custom TUI components for interactive user interfaces. This page covers the component system and available building blocks.

**Source:** [`@dreb/tui`](https://github.com/aebrer/dreb/tree/master/packages/tui)

## Component Interface

All components implement:

```typescript
interface Component {
  render(width: number): string[];
  handleInput?(data: string): void;
  wantsKeyRelease?: boolean;
  invalidate(): void;
}
```

| Method | Description |
|--------|-------------|
| `render(width)` | Return array of strings (one per line). Each line **must not exceed `width`**. |
| `handleInput?(data)` | Receive keyboard input when component has focus. |
| `wantsKeyRelease?` | If true, component receives key release events (Kitty protocol). Default: false. |
| `invalidate()` | Clear cached render state. Called on theme changes. |

The TUI appends a full SGR reset and OSC 8 reset at the end of each rendered line. Styles do not carry across lines. If you emit multi-line text with styling, reapply styles per line or use `wrapTextWithAnsi()` so styles are preserved for each wrapped line.

## Focusable Interface (IME Support)

Components that display a text cursor and need IME (Input Method Editor) support should implement the `Focusable` interface:

```typescript
import { CURSOR_MARKER, type Component, type Focusable } from "@dreb/tui";

class MyInput implements Component, Focusable {
  focused: boolean = false;  // Set by TUI when focus changes
  
  render(width: number): string[] {
    const marker = this.focused ? CURSOR_MARKER : "";
    // Emit marker right before the fake cursor
    return [`> ${beforeCursor}${marker}\x1b[7m${atCursor}\x1b[27m${afterCursor}`];
  }
}
```

When a `Focusable` component has focus, TUI:
1. Sets `focused = true` on the component
2. Scans rendered output for `CURSOR_MARKER` (a zero-width APC escape sequence)
3. Positions the hardware terminal cursor at that location
4. Shows the hardware cursor

This enables IME candidate windows to appear at the correct position for CJK input methods. The `Editor` and `Input` built-in components already implement this interface.

### Container Components with Embedded Inputs

When a container component (dialog, selector, etc.) contains an `Input` or `Editor` child, the container must implement `Focusable` and propagate the focus state to the child. Otherwise, the hardware cursor won't be positioned correctly for IME input.

```typescript
import { Container, type Focusable, Input } from "@dreb/tui";

class SearchDialog extends Container implements Focusable {
  private searchInput: Input;

  // Focusable implementation - propagate to child input for IME cursor positioning
  private _focused = false;
  get focused(): boolean {
    return this._focused;
  }
  set focused(value: boolean) {
    this._focused = value;
    this.searchInput.focused = value;
  }

  constructor() {
    super();
    this.searchInput = new Input();
    this.addChild(this.searchInput);
  }
}
```

Without this propagation, typing with an IME (Chinese, Japanese, Korean, etc.) will show the candidate window in the wrong position on screen.

## Using Components

**In extensions** via `ctx.ui.custom()`:

```typescript
dreb.on("session_start", async (_event, ctx) => {
  const handle = ctx.ui.custom(myComponent);
  // handle.requestRender() - trigger re-render
  // handle.close() - restore normal UI
});
```

**In custom tools** via `dreb.ui.custom()`:

```typescript
async execute(toolCallId, params, onUpdate, ctx, signal) {
  const handle = dreb.ui.custom(myComponent);
  // ...
  handle.close();
}
```

## Overlays

Overlays render components on top of existing content without clearing the screen. Pass `{ overlay: true }` to `ctx.ui.custom()`:

```typescript
const result = await ctx.ui.custom<string | null>(
  (tui, theme, keybindings, done) => new MyDialog({ onClose: done }),
  { overlay: true }
);
```

For positioning and sizing, use `overlayOptions`:

```typescript
const result = await ctx.ui.custom<string | null>(
  (tui, theme, keybindings, done) => new SidePanel({ onClose: done }),
  {
    overlay: true,
    overlayOptions: {
      // Size: number or percentage string
      width: "50%",          // 50% of terminal width
      minWidth: 40,          // minimum 40 columns
      maxHeight: "80%",      // max 80% of terminal height

      // Position: anchor-based (default: "center")
      anchor: "right-center", // 9 positions: center, top-left, top-center, etc.
      offsetX: -2,            // offset from anchor
      offsetY: 0,

      // Or percentage/absolute positioning
      row: "25%",            // 25% from top
      col: 10,               // column 10

      // Margins
      margin: 2,             // all sides, or { top, right, bottom, left }

      // Responsive: hide on narrow terminals
      visible: (termWidth, termHeight) => termWidth >= 80,
    },
    // Get handle for programmatic visibility control
    onHandle: (handle) => {
      // handle.setHidden(true/false) - toggle visibility
      // handle.hide() - permanently remove
    },
  }
);
```

### Overlay Lifecycle

Overlay components are disposed when closed. Don't reuse references - create fresh instances:

```typescript
// Wrong - stale reference
let menu: MenuComponent;
await ctx.ui.custom((_, __, ___, done) => {
  menu = new MenuComponent(done);
  return menu;
}, { overlay: true });
setActiveComponent(menu);  // Disposed

// Correct - re-call to re-show
const showMenu = () => ctx.ui.custom((_, __, ___, done) => 
  new MenuComponent(done), { overlay: true });

await showMenu();  // First show
await showMenu();  // "Back" = just call again
```

See [overlay-qa-tests.ts](../examples/extensions/overlay-qa-tests.ts) for comprehensive examples covering anchors, margins, stacking, responsive visibility, and animation.

## Built-in Components

Import from `@dreb/tui`:

```typescript
import { Text, Box, Container, Spacer, Markdown } from "@dreb/tui";
```

### Text

Multi-line text with word wrapping.

```typescript
const text = new Text(
  "Hello World",    // content
  1,                // paddingX (default: 1)
  1,                // paddingY (default: 1)
  (s) => bgGray(s)  // optional background function
);
text.setText("Updated");
```

### Box

Container with padding and background color.

```typescript
const box = new Box(
  1,                // paddingX
  1,                // paddingY
  (s) => bgGray(s)  // background function
);
box.addChild(new Text("Content", 0, 0));
box.setBgFn((s) => bgBlue(s));
```

### Container

Groups child components vertically.

```typescript
const container = new Container();
container.addChild(component1);
container.addChild(component2);
container.removeChild(component1);
```

### Spacer

Empty vertical space.

```typescript
const spacer = new Spacer(2);  // 2 empty lines
```

### Markdown

Renders markdown with syntax highlighting.

```typescript
const md = new Markdown(
  "# Title\n\nSome **bold** text",
  1,        // paddingX
  1,        // paddingY
  theme     // MarkdownTheme (see below)
);
md.setText("Updated markdown");
```

### Image

Renders images in supported terminals (Kitty, iTerm2, Ghostty, WezTerm).

```typescript
const image = new Image(
  base64Data,   // base64-encoded image
  "image/png",  // MIME type
  theme,        // ImageTheme
  { maxWidthCells: 80, maxHeightCells: 24 }
);
```

## Keyboard Input

Use `matchesKey()` for key detection:

```typescript
import { matchesKey, Key } from "@dreb/tui";

handleInput(data: string) {
  if (matchesKey(data, Key.up)) {
    this.selectedIndex--;
  } else if (matchesKey(data, Key.enter)) {
    this.onSelect?.(this.selectedIndex);
  } else if (matchesKey(data, Key.escape)) {
    this.onCancel?.();
  } else if (matchesKey(data, Key.ctrl("c"))) {
    // Ctrl+C
  }
}
```

**Key identifiers** (use `Key.*` for autocomplete, or string literals):
- Basic keys: `Key.enter`, `Key.escape`, `Key.tab`, `Key.space`, `Key.backspace`, `Key.delete`, `Key.home`, `Key.end`
- Arrow keys: `Key.up`, `Key.down`, `Key.left`, `Key.right`
- With modifiers: `Key.ctrl("c")`, `Key.shift("tab")`, `Key.alt("left")`, `Key.ctrlShift("p")`
- String format also works: `"enter"`, `"ctrl+c"`, `"shift+tab"`, `"ctrl+shift+p"`

## Line Width

**Critical:** Each line from `render()` must not exceed the `width` parameter.

```typescript
import { visibleWidth, truncateToWidth } from "@dreb/tui";

render(width: number): string[] {
  // Truncate long lines
  return [truncateToWidth(this.text, width)];
}
```

Utilities:
- `visibleWidth(str)` - Get display width (ignores ANSI codes)
- `truncateToWidth(str, width, ellipsis?)` - Truncate with optional ellipsis
- `wrapTextWithAnsi(str, width)` - Word wrap preserving ANSI codes

## Creating Custom Components

Example: Interactive selector

```typescript
import {
  matchesKey, Key,
  truncateToWidth, visibleWidth
} from "@dreb/tui";

class MySelector {
  private items: string[];
  private selected = 0;
  private cachedWidth?: number;
  private cachedLines?: string[];
  
  public onSelect?: (item: string) => void;
  public onCancel?: () => void;

  constructor(items: string[]) {
    this.items = items;
  }

  handleInput(data: string): void {
    if (matchesKey(data, Key.up) && this.selected > 0) {
      this.selected--;
      this.invalidate();
    } else if (matchesKey(data, Key.down) && this.selected < this.items.length - 1) {
      this.selected++;
      this.invalidate();
    } else if (matchesKey(data, Key.enter)) {
      this.onSelect?.(this.items[this.selected]);
    } else if (matchesKey(data, Key.escape)) {
      this.onCancel?.();
    }
  }

  render(width: number): string[] {
    if (this.cachedLines && this.cachedWidth === width) {
      return this.cachedLines;
    }

    this.cachedLines = this.items.map((item, i) => {
      const prefix = i === this.selected ? "> " : "  ";
      return truncateToWidth(prefix + item, width);
    });
    this.cachedWidth = width;
    return this.cachedLines;
  }

  invalidate(): void {
    this.cachedWidth = undefined;
    this.cachedLines = undefined;
  }
}
```

Usage in an extension:

```typescript
dreb.registerCommand("pick", {
  description: "Pick an item",
  handler: async (args, ctx) => {
    const items = ["Option A", "Option B", "Option C"];
    const selector = new MySelector(items);
    
    let handle: { close: () => void; requestRender: () => void };
    
    await new Promise<void>((resolve) => {
      selector.onSelect = (item) => {
        ctx.ui.notify(`Selected: ${item}`, "info");
        handle.close();
        resolve();
      };
      selector.onCancel = () => {
        handle.close();
        resolve();
      };
      handle = ctx.ui.custom(selector);
    });
  }
});
```

## Theming

Components accept theme objects for styling.

**In `renderCall`/`renderResult`**, use the `theme` parameter:

```typescript
renderResult(result, options, theme, context) {
  // Use theme.fg() for foreground colors
  return new Text(theme.fg("success", "Done!"), 0, 0);
  
  // Use theme.bg() for background colors
  const styled = theme.bg("toolPendingBg", theme.fg("accent", "text"));
}
```

**Foreground colors** (`theme.fg(color, text)`):

| Category | Colors |
|----------|--------|
| General | `text`, `accent`, `muted`, `dim` |
| Status | `success`, `error`, `warning` |
| Borders | `border`, `borderAccent`, `borderMuted` |
| Messages | `userMessageText`, `customMessageText`, `customMessageLabel` |
| Tools | `toolTitle`, `toolOutput` |
| Diffs | `toolDiffAdded`, `toolDiffRemoved`, `toolDiffContext` |
| Markdown | `mdHeading`, `mdLink`, `mdLinkUrl`, `mdCode`, `mdCodeBlock`, `mdCodeBlockBorder`, `mdQuote`, `mdQuoteBorder`, `mdHr`, `mdListBullet` |
| Syntax | `syntaxComment`, `syntaxKeyword`, `syntaxFunction`, `syntaxVariable`, `syntaxString`, `syntaxNumber`, `syntaxType`, `syntaxOperator`, `syntaxPunctuation` |
| Thinking | `thinkingOff`, `thinkingMinimal`, `thinkingLow`, `thinkingMedium`, `thinkingHigh`, `thinkingXhigh` |
| Modes | `bashMode` |

**Background colors** (`theme.bg(color, text)`):

`selectedBg`, `userMessageBg`, `customMessageBg`, `toolPendingBg`, `toolSuccessBg`, `toolErrorBg`

**For Markdown**, use `getMarkdownTheme()`:

```typescript
import { getMarkdownTheme } from "@dreb/coding-agent";
import { Markdown } from "@dreb/tui";

renderResult(result, options, theme, context) {
  const mdTheme = getMarkdownTheme();
  return new Markdown(result.details.markdown, 0, 0, mdTheme);
}
```

**For custom components**, define your own theme interface:

```typescript
interface MyTheme {
  selected: (s: string) => string;
  normal: (s: string) => string;
}
```

## Debug logging

Set `DREB_TUI_WRITE_LOG` to capture the raw ANSI stream written to stdout.

```bash
DREB_TUI_WRITE_LOG=/tmp/tui-ansi.log npx tsx packages/tui/test/chat-simple.ts
```

## Performance

Cache rendered output when possible:

```typescript
class CachedComponent {
  private cachedWidth?: number;
  private cachedLines?: string[];

  render(width: number): string[] {
    if (this.cachedLines && this.cachedWidth === width) {
      return this.cachedLines;
    }
    // ... compute lines ...
    this.cachedWidth = width;
    this.cachedLines = lines;
    return lines;
  }

  invalidate(): void {
    this.cachedWidth = undefined;
    this.cachedLines = undefined;
  }
}
```

Call `invalidate()` when state changes, then `handle.requestRender()` to trigger re-render.

## Invalidation and Theme Changes

When the theme changes, the TUI calls `invalidate()` on all components to clear their caches. Components must properly implement `invalidate()` to ensure theme changes take effect.

### The Problem

If a component pre-bakes theme colors into strings (via `theme.fg()`, `theme.bg()`, etc.) and caches them, the cached strings contain ANSI escape codes from the old theme. Simply clearing the render cache isn't enough if the component stores the themed content separately.

**Wrong approach** (theme colors won't update):

```typescript
class BadComponent extends Container {
  private content: Text;

  constructor(message: string, theme: Theme) {
    super();
    // Pre-baked theme colors stored in Text component
    this.content = new Text(theme.fg("accent", message), 1, 0);
    this.addChild(this.content);
  }
  // No invalidate override - parent's invalidate only clears
  // child render caches, not the pre-baked content
}
```

### The Solution

Components that build content with theme colors must rebuild that content when `invalidate()` is called:

```typescript
class GoodComponent extends Container {
  private message: string;
  private content: Text;

  constructor(message: string) {
    super();
    this.message = message;
    this.content = new Text("", 1, 0);
    this.addChild(this.content);
    this.updateDisplay();
  }

  private updateDisplay(): void {
    // Rebuild content with current theme
    this.content.setText(theme.fg("accent", this.message));
  }

  override invalidate(): void {
    super.invalidate();  // Clear child caches
    this.updateDisplay(); // Rebuild with new theme
  }
}
```

### Pattern: Rebuild on Invalidate

For components with complex content:

```typescript
class ComplexComponent extends Container {
  private data: SomeData;

  constructor(data: SomeData) {
    super();
    this.data = data;
    this.rebuild();
  }

  private rebuild(): void {
    this.clear();  // Remove all children

    // Build UI with current theme
    this.addChild(new Text(theme.fg("accent", theme.bold("Title")), 1, 0));
    this.addChild(new Spacer(1));

    for (const item of this.data.items) {
      const color = item.active ? "success" : "muted";
      this.addChild(new Text(theme.fg(color, item.label), 1, 0));
    }
  }

  override invalidate(): void {
    super.invalidate();
    this.rebuild();
  }
}
```

### When This Matters

This pattern is needed when:

1. **Pre-baking theme colors** - Using `theme.fg()` or `theme.bg()` to create styled strings stored in child components
2. **Syntax highlighting** - Using `highlightCode()` which applies theme-based syntax colors
3. **Complex layouts** - Building child component trees that embed theme colors

This pattern is NOT needed when:

1. **Using theme callbacks** - Passing functions like `(text) => theme.fg("accent", text)` that are called during render
2. **Simple containers** - Just grouping other components without adding themed content
3. **Stateless render** - Computing themed output fresh in every `render()` call (no caching)

## Common Patterns

These patterns cover the most common UI needs in extensions. **Copy these patterns instead of building from scratch.**

### Pattern 1: Selection Dialog (SelectList)

For letting users pick from a list of options. Use `SelectList` from `@dreb/tui` with `DynamicBorder` for framing.

```typescript
import type { ExtensionAPI } from "@dreb/coding-agent";
import { DynamicBorder } from "@dreb/coding-agent";
import { Container, type SelectItem, SelectList, Text } from "@dreb/tui";

dreb.registerCommand("pick", {
  handler: async (_args, ctx) => {
    const items: SelectItem[] = [
      { value: "opt1", label: "Option 1", description: "First option" },
      { value: "opt2", label: "Option 2", description: "Second option" },
      { value: "opt3", label: "Option 3" },  // description is optional
    ];

    const result = await ctx.ui.custom<string | null>((tui, theme, _kb, done) => {
      const container = new Container();

      // Top border
      container.addChild(new DynamicBorder((s: string) => theme.fg("accent", s)));

      // Title
      container.addChild(new Text(theme.fg("accent", theme.bold("Pick an Option")), 1, 0));

      // SelectList with theme
      const selectList = new SelectList(items, Math.min(items.length, 10), {
        selectedPrefix: (t) => theme.fg("accent", t),
        selectedText: (t) => theme.fg("accent", t),
        description: (t) => theme.fg("muted", t),
        scrollInfo: (t) => theme.fg("dim", t),
        noMatch: (t) => theme.fg("warning", t),
      });
      selectList.onSelect = (item) => done(item.value);
      selectList.onCancel = () => done(null);
      container.addChild(selectList);

      // Help text
      container.addChild(new Text(theme.fg("dim", "↑↓ navigate • enter select • esc cancel"), 1, 0));

      // Bottom border
      container.addChild(new DynamicBorder((s: string) => theme.fg("accent", s)));

      return {
        render: (w) => container.render(w),
        invalidate: () => container.invalidate(),
        handleInput: (data) => { selectList.handleInput(data); tui.requestRender(); },
      };
    });

    if (result) {
      ctx.ui.notify(`Selected: ${result}`, "info");
    }
  },
});
```

**Examples:** [preset.ts](../examples/extensions/preset.ts), [tools.ts](../examples/extensions/tools.ts)

### Pattern 2: Async Operation with Cancel (BorderedLoader)

For operations that take time and should be cancellable. `BorderedLoader` shows a spinner and handles escape to cancel.

```typescript
import { BorderedLoader } from "@dreb/coding-agent";

dreb.registerCommand("fetch", {
  handler: async (_args, ctx) => {
    const result = await ctx.ui.custom<string | null>((tui, theme, _kb, done) => {
      const loader = new BorderedLoader(tui, theme, "Fetching data...");
      loader.onAbort = () => done(null);

      // Do async work
      fetchData(loader.signal)
        .then((data) => done(data))
        .catch(() => done(null));

      return loader;
    });

    if (result === null) {
      ctx.ui.notify("Cancelled", "info");
    } else {
      ctx.ui.setEditorText(result);
    }
  },
});
```

**Examples:** [qna.ts](../examples/extensions/qna.ts), [handoff.ts](../examples/extensions/handoff.ts)

### Pattern 3: Settings/Toggles (SettingsList)

For toggling multiple settings. Use `SettingsList` from `@dreb/tui` with `getSettingsListTheme()`.

```typescript
import { getSettingsListTheme } from "@dreb/coding-agent";
import { Container, type SettingItem, SettingsList, Text } from "@dreb/tui";

dreb.registerCommand("settings", {
  handler: async (_args, ctx) => {
    const items: SettingItem[] = [
      { id: "verbose", label: "Verbose mode", currentValue: "off", values: ["on", "off"] },
      { id: "color", label: "Color output", currentValue: "on", values: ["on", "off"] },
    ];

    await ctx.ui.custom((_tui, theme, _kb, done) => {
      const container = new Container();
      container.addChild(new Text(theme.fg("accent", theme.bold("Settings")), 1, 1));

      const settingsList = new SettingsList(
        items,
        Math.min(items.length + 2, 15),
        getSettingsListTheme(),
        (id, newValue) => {
          // Handle value change
          ctx.ui.notify(`${id} = ${newValue}`, "info");
        },
        () => done(undefined),  // On close
        { enableSearch: true }, // Optional: enable fuzzy search by label
      );
      container.addChild(settingsList);

      return {
        render: (w) => container.render(w),
        invalidate: () => container.invalidate(),
        handleInput: (data) => settingsList.handleInput?.(data),
      };
    });
  },
});
```

**Examples:** [tools.ts](../examples/extensions/tools.ts)

### Pattern 4: Persistent Status Indicator

Show status in the footer that persists across renders. Good for mode indicators.

```typescript
// Set status (shown in footer)
ctx.ui.setStatus("my-ext", ctx.ui.theme.fg("accent", "● active"));

// Clear status
ctx.ui.setStatus("my-ext", undefined);
```

**Examples:** [status-line.ts](../examples/extensions/status-line.ts), [plan-mode.ts](../examples/extensions/plan-mode.ts), [preset.ts](../examples/extensions/preset.ts)

### Pattern 5: Widgets Above/Below Editor

Show persistent content above or below the input editor. Good for todo lists, progress.

```typescript
// Simple string array (above editor by default)
ctx.ui.setWidget("my-widget", ["Line 1", "Line 2"]);

// Render below the editor
ctx.ui.setWidget("my-widget", ["Line 1", "Line 2"], { placement: "belowEditor" });

// Or with theme
ctx.ui.setWidget("my-widget", (_tui, theme) => {
  const lines = items.map((item, i) =>
    item.done
      ? theme.fg("success", "✓ ") + theme.fg("muted", item.text)
      : theme.fg("dim", "○ ") + item.text
  );
  return {
    render: () => lines,
    invalidate: () => {},
  };
});

// Clear
ctx.ui.setWidget("my-widget", undefined);
```

**Examples:** [plan-mode.ts](../examples/extensions/plan-mode.ts)

### Pattern 6: Custom Footer

Replace the footer. `footerData` exposes data not otherwise accessible to extensions.

```typescript
ctx.ui.setFooter((tui, theme, footerData) => ({
  invalidate() {},
  render(width: number): string[] {
    // footerData.getGitBranch(): string | null
    // footerData.getExtensionStatuses(): ReadonlyMap<string, string>
    // footerData.getDailyCost(): number — aggregate cost across all sessions today
    return [`${ctx.model?.id} (${footerData.getGitBranch() || "no git"})`];
  },
  dispose: footerData.onBranchChange(() => tui.requestRender()), // reactive
}));

ctx.ui.setFooter(undefined); // restore default
```

Token stats available via `ctx.sessionManager.getBranch()` and `ctx.model`.

**Examples:** [custom-footer.ts](../examples/extensions/custom-footer.ts)

### Pattern 7: Custom Editor (vim mode, etc.)

Replace the main input editor with a custom implementation. Useful for modal editing (vim), different keybindings (emacs), or specialized input handling.

```typescript
import { CustomEditor, type ExtensionAPI } from "@dreb/coding-agent";
import { matchesKey, truncateToWidth } from "@dreb/tui";

type Mode = "normal" | "insert";

class VimEditor extends CustomEditor {
  private mode: Mode = "insert";

  handleInput(data: string): void {
    // Escape: switch to normal mode, or pass through for app handling
    if (matchesKey(data, "escape")) {
      if (this.mode === "insert") {
        this.mode = "normal";
        return;
      }
      // In normal mode, escape aborts agent (handled by CustomEditor)
      super.handleInput(data);
      return;
    }

    // Insert mode: pass everything to CustomEditor
    if (this.mode === "insert") {
      super.handleInput(data);
      return;
    }

    // Normal mode: vim-style navigation
    switch (data) {
      case "i": this.mode = "insert"; return;
      case "h": super.handleInput("\x1b[D"); return; // Left
      case "j": super.handleInput("\x1b[B"); return; // Down
      case "k": super.handleInput("\x1b[A"); return; // Up
      case "l": super.handleInput("\x1b[C"); return; // Right
    }
    // Pass unhandled keys to super (ctrl+c, etc.), but filter printable chars
    if (data.length === 1 && data.charCodeAt(0) >= 32) return;
    super.handleInput(data);
  }

  render(width: number): string[] {
    const lines = super.render(width);
    // Add mode indicator to bottom border (use truncateToWidth for ANSI-safe truncation)
    if (lines.length > 0) {
      const label = this.mode === "normal" ? " NORMAL " : " INSERT ";
      const lastLine = lines[lines.length - 1]!;
      // Pass "" as ellipsis to avoid adding "..." when truncating
      lines[lines.length - 1] = truncateToWidth(lastLine, width - label.length, "") + label;
    }
    return lines;
  }
}

export default function (dreb: ExtensionAPI) {
  dreb.on("session_start", (_event, ctx) => {
    // Factory receives theme and keybindings from the app
    ctx.ui.setEditorComponent((tui, theme, keybindings) =>
      new VimEditor(theme, keybindings)
    );
  });
}
```

**Key points:**

- **Extend `CustomEditor`** (not base `Editor`) to get app keybindings (escape to abort, ctrl+d to exit, model switching, etc.)
- **Call `super.handleInput(data)`** for keys you don't handle
- **Factory pattern**: `setEditorComponent` receives a factory function that gets `tui`, `theme`, and `keybindings`
- **Pass `undefined`** to restore the default editor: `ctx.ui.setEditorComponent(undefined)`

**Examples:** [modal-editor.ts](../examples/extensions/modal-editor.ts)

## Key Rules

1. **Always use theme from callback** - Don't import theme directly. Use `theme` from the `ctx.ui.custom((tui, theme, keybindings, done) => ...)` callback.

2. **Always type DynamicBorder color param** - Write `(s: string) => theme.fg("accent", s)`, not `(s) => theme.fg("accent", s)`.

3. **Call tui.requestRender() after state changes** - In `handleInput`, call `tui.requestRender()` after updating state.

4. **Return the three-method object** - Custom components need `{ render, invalidate, handleInput }`.

5. **Use existing components** - `SelectList`, `SettingsList`, `BorderedLoader` cover 90% of cases. Don't rebuild them.

## Examples

- **Selection UI**: [examples/extensions/preset.ts](../examples/extensions/preset.ts) - SelectList with DynamicBorder framing
- **Async with cancel**: [examples/extensions/qna.ts](../examples/extensions/qna.ts) - BorderedLoader for LLM calls
- **Settings toggles**: [examples/extensions/tools.ts](../examples/extensions/tools.ts) - SettingsList for tool enable/disable
- **Status indicators**: [examples/extensions/plan-mode.ts](../examples/extensions/plan-mode.ts) - setStatus and setWidget
- **Custom footer**: [examples/extensions/custom-footer.ts](../examples/extensions/custom-footer.ts) - setFooter with stats
- **Custom editor**: [examples/extensions/modal-editor.ts](../examples/extensions/modal-editor.ts) - Vim-like modal editing
- **Snake game**: [examples/extensions/snake.ts](../examples/extensions/snake.ts) - Full game with keyboard input, game loop
- **Custom tool rendering**: [examples/extensions/todo.ts](../examples/extensions/todo.ts) - renderCall and renderResult
