Test palm rejection for add-page button taps

Move the draw-or-navigate choice into touchIntent() so the rule the
canvas uses can be tested, and cover finger taps on the plus button:
accepted after writing and while the pen hovers, rejected while the
pen is down, for palm-sized contacts and right after pen activity.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GaoZtJQZwLXkEgWs8twCia

src/lib/components/CanvasView.svelte

@@ -16,7 +16,7 @@
 	import { bounds, intersects, rectBeside, type Rect } from '$lib/model/layout';
 	import { canInsert, freeSlots, freeSlotsOf, neighbor, type Dir, type Slot } from '$lib/model/tree';
 	import { A4, PAGE_PRESETS, type Stroke } from '$lib/model/types';
-	import { PalmRejector } from '$lib/ink/palm';
+	import { PalmRejector, touchIntent } from '$lib/ink/palm';
 	import { StrokeRecorder, hitStroke } from '$lib/ink/stroke';
 	import { pageCssVars } from '$lib/editor/pageStyle';
 	import { downloadCanvas } from './download';
@@ -301,7 +301,7 @@
 		// pen-down, palm-sized contacts and right after pen use, but not for the 5 s pen session.
 		const plus = target.closest<HTMLElement>('.ghost');
 		if (e.pointerType === 'touch') {
-			const decision = palm.evaluate(e, fingerInks && !plus ? 'draw' : 'navigate');
+			const decision = palm.evaluate(e, touchIntent(fingerInks, !!plus));
 			if (!decision.accept) return; // palm: ignore entirely
 			touches.set(e.pointerId, p);
 			if (touches.size >= 2) {

src/lib/ink/palm.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
-import { PalmRejector } from './palm';
+import { PalmRejector, touchIntent } from './palm';
 
 const ev = (type: string, pointerType: string, extra: Partial<PointerEvent> = {}) =>
 	({ type, pointerType, pointerId: pointerType === 'pen' ? 1 : 2, width: 10, height: 10, ...extra }) as PointerEvent;
@@ -53,4 +53,37 @@ describe('palm rejection', () => {
 	it('allows finger ink again once the pen session ends', () => {
 		expect(afterPen(5100).evaluate(touch(), 'draw').accept).toBe(true);
 	});
+
+	describe('add-page button taps', () => {
+		// With a pen in use fingers don't ink, but finger ink may still be on (before any pen).
+		const tap = (palm: PalmRejector, extra: Partial<PointerEvent> = {}, fingerInks = false) =>
+			palm.evaluate(touch(extra), touchIntent(fingerInks, true));
+
+		it('are handled as navigation even when fingers ink', () => {
+			expect(touchIntent(true, true)).toBe('navigate');
+			expect(touchIntent(false, true)).toBe('navigate');
+			expect(touchIntent(true, false)).toBe('draw');
+			expect(touchIntent(false, false)).toBe('navigate');
+		});
+
+		it('are accepted a couple of seconds after writing', () => {
+			expect(tap(afterPen(2000)).accept).toBe(true);
+			expect(tap(afterPen(2000), {}, true).accept).toBe(true);
+		});
+
+		it('are accepted while the pen hovers', () => {
+			expect(tap(afterPen(2000, { leave: false })).accept).toBe(true);
+		});
+
+		it('are rejected while the pen is down', () => {
+			const palm = new PalmRejector();
+			palm.track(ev('pointerdown', 'pen'));
+			expect(tap(palm).accept).toBe(false);
+		});
+
+		it('are rejected for a palm or right after the pen', () => {
+			expect(tap(afterPen(2000), { width: 60, height: 40 }).accept).toBe(false);
+			expect(tap(afterPen(50)).accept).toBe(false);
+		});
+	});
 });

src/lib/ink/palm.ts

@@ -27,6 +27,15 @@ const PALM_CONTACT_PX = 35;
 /** What an accepted touch would do: ink (draw/erase) or pan/zoom. */
 export type TouchIntent = 'draw' | 'navigate';
 
+/**
+ * Which policy a touch falls under. It only draws when finger ink is on and it
+ * didn't land on a button; taps on buttons (e.g. add page) navigate, so they keep
+ * working between pen strokes.
+ */
+export function touchIntent(fingerInks: boolean, onButton: boolean): TouchIntent {
+	return fingerInks && !onButton ? 'draw' : 'navigate';
+}
+
 export interface Decision {
 	accept: boolean;
 	reason: string;