feat: add dynamic font resize support

Implement runtime font size and family changes via options proxy,
matching xterm.js API behavior.

Changes:
- Wire up fontSize option changes to renderer.setFontSize()
- Wire up fontFamily option changes to renderer.setFontFamily()
- Add handleFontChange() helper that:
  - Clears active selection (pixel positions changed)
  - Resizes canvas to match new font metrics
  - Forces full re-render
- Add tests for font change behavior

Usage:
  term.options.fontSize = 20;  // Dynamic resize
  term.options.fontFamily = 'Courier New';

Note: Font changes keep cols/rows constant (canvas grows/shrinks).
Use FitAddon.fit() after font change to recalculate grid dimensions.

Author: sreya

lib/terminal.test.ts

@@ -2462,6 +2462,98 @@ describe('Options Proxy handleOptionChange', () => {
 
     expect(term.options.cursorStyle).toBe('underline');
   });
+
+  test('changing fontSize updates renderer and resizes canvas', async () => {
+    if (!container) return;
+
+    const term = await createIsolatedTerminal({ fontSize: 15, cols: 80, rows: 24 });
+    term.open(container);
+
+    // Get initial metrics
+    // @ts-ignore - accessing private for test
+    const renderer = term.renderer;
+    const initialMetrics = renderer.getMetrics();
+    // @ts-ignore - accessing private for test
+    const canvas = term.canvas;
+    const initialWidth = canvas.style.width;
+
+    // Change font size
+    term.options.fontSize = 20;
+
+    // Verify option was updated
+    expect(term.options.fontSize).toBe(20);
+
+    // Verify renderer metrics changed
+    const newMetrics = renderer.getMetrics();
+    expect(newMetrics.width).toBeGreaterThan(initialMetrics.width);
+    expect(newMetrics.height).toBeGreaterThan(initialMetrics.height);
+
+    // Verify canvas size changed
+    expect(canvas.style.width).not.toBe(initialWidth);
+
+    term.dispose();
+  });
+
+  test('changing fontFamily updates renderer', async () => {
+    if (!container) return;
+
+    const term = await createIsolatedTerminal({ fontFamily: 'monospace', cols: 80, rows: 24 });
+    term.open(container);
+
+    // @ts-ignore - accessing private for test
+    const renderer = term.renderer;
+
+    // Change font family
+    term.options.fontFamily = 'Courier New, monospace';
+
+    // Verify option was updated
+    expect(term.options.fontFamily).toBe('Courier New, monospace');
+
+    // Verify renderer was updated
+    // @ts-ignore - accessing private for test
+    expect(renderer.fontFamily).toBe('Courier New, monospace');
+
+    term.dispose();
+  });
+
+  test('font change clears active selection', async () => {
+    if (!container) return;
+
+    const term = await createIsolatedTerminal({ fontSize: 15, cols: 80, rows: 24 });
+    term.open(container);
+
+    // Write some text and select it
+    term.write('Hello World');
+    term.select(0, 0, 5); // Select "Hello"
+    expect(term.hasSelection()).toBe(true);
+
+    // Change font size
+    term.options.fontSize = 20;
+
+    // Selection should be cleared (pixel positions changed)
+    expect(term.hasSelection()).toBe(false);
+
+    term.dispose();
+  });
+
+  test('font change maintains terminal dimensions (cols/rows)', async () => {
+    if (!container) return;
+
+    const term = await createIsolatedTerminal({ fontSize: 15, cols: 80, rows: 24 });
+    term.open(container);
+
+    const initialCols = term.cols;
+    const initialRows = term.rows;
+
+    // Change font size
+    term.options.fontSize = 20;
+
+    // Cols and rows should remain the same (canvas grows instead)
+    expect(term.cols).toBe(initialCols);
+    expect(term.rows).toBe(initialRows);
+
+    term.dispose();
+  });
 });
 
 // ==========================================================================

lib/terminal.ts

@@ -206,9 +206,16 @@ export class Terminal implements ITerminalCore {
         break;
 
       case 'fontSize':
+        if (this.renderer) {
+          this.renderer.setFontSize(this.options.fontSize);
+          this.handleFontChange();
+        }
+        break;
+
       case 'fontFamily':
         if (this.renderer) {
-          console.warn('ghostty-web: font changes after open() are not yet fully supported');
+          this.renderer.setFontFamily(this.options.fontFamily);
+          this.handleFontChange();
         }
         break;
 
@@ -220,6 +227,32 @@ export class Terminal implements ITerminalCore {
     }
   }
 
+  /**
+   * Handle font changes (fontSize or fontFamily)
+   * Updates canvas size to match new font metrics and forces a full re-render
+   */
+  private handleFontChange(): void {
+    if (!this.renderer || !this.wasmTerm || !this.canvas) return;
+
+    // Clear any active selection since pixel positions have changed
+    if (this.selectionManager) {
+      this.selectionManager.clearSelection();
+    }
+
+    // Resize canvas to match new font metrics
+    this.renderer.resize(this.cols, this.rows);
+
+    // Update canvas element dimensions to match renderer
+    const metrics = this.renderer.getMetrics();
+    this.canvas.width = metrics.width * this.cols;
+    this.canvas.height = metrics.height * this.rows;
+    this.canvas.style.width = `${metrics.width * this.cols}px`;
+    this.canvas.style.height = `${metrics.height * this.rows}px`;
+
+    // Force full re-render with new font
+    this.renderer.render(this.wasmTerm, true, this.viewportY, this);
+  }
+
   /**
    * Parse a CSS color string to 0xRRGGBB format.
    * Returns 0 if the color is undefined or invalid.