Merge pull request 'Let the page flick work anywhere, with a setting to turn it off' (#13) from canvas-polish into main

Reviewed-on: #13

README.md

@@ -23,7 +23,8 @@ The image builds the PWA and serves it with nginx. Put an HTTPS reverse proxy in
 ## Using it
 
 - **Tools** (bottom bar): Text `T`, Pen `P`, Highlighter `H`, Eraser `E`, Import PDF, Undo/Redo, zoom. The pen's eraser end erases too.
-- **Pages**: the grey "+" tiles next to the active page add a page there; `Alt+Arrow` moves to (or creates) the neighbour. Right-click a page or use its `⋯` button to insert, resize or delete it.
+- **Pages**: the "+" circles beside the active page add a page there (a quick finger flick toward a free side offers one there; turn it off under Settings → Pen & touch); `Alt+Arrow` moves to (or creates) the neighbour. Right-click a page or use its `⋯` button to insert, resize or delete it.
+- **Grid**: the faint background grid has one cell per active page; Settings → Appearance can hide it or subdivide it.
 - **Importing a PDF** into a non-empty canvas shows every free spot; click one to place the chain. Into a blank canvas it simply replaces the empty page. The Files panel can also import a PDF as a new canvas.
 - **View**: wheel/trackpad pans, `Ctrl`+wheel or pinch zooms, `Ctrl+0` fits the page, `Ctrl+9` shows the whole tree, space-drag or middle-drag pans.
 - **Sync**: Settings → Sync. Use a fine-grained token with *Contents: read & write* on the repo. Edits autosave locally; pushes happen every N minutes and on `Ctrl+S` / the sync button. Conflicts: newer wins.

spec.md

@@ -9,10 +9,11 @@ One canvas = one tree = one `.pdf` file.
   - A branch can only ever continue in the direction it started (strictly linear — no sub-branches off a branch).
   - A trunk page can have at most one left branch and one right branch.
 - **PDF import**: creates a chain of pages in whatever direction you pick when placing it (up/down = extends the trunk, left/right = becomes a branch off the trunk page you clicked). Only one trunk exists per canvas — additional imported PDFs must attach as a branch off some trunk page, or extend an existing branch further outward in its established direction.
-- Hidden/unopened neighbor pages render as a gray tile with a "+" in a circle; clicking (or a shortcut) instantiates them as real pages.
+- Each free side of the active page shows a small "+" circle just off its edge; tapping it (or `Alt+Arrow`) adds a blank page there. A quick finger fling toward a free side offers a page there for a few seconds: that side's "+" pulses, or, if it is off screen, an "Add page" button appears at that edge of the screen. The fling never adds a page by itself, and can be turned off in Settings.
+- The canvas background is a faint grid with one cell per active page, aligned to its edges; Settings can hide it or subdivide it.
 - Deleting a page removes it (undo via Ctrl+Z); no permanent trash.
 - Default zoom: one page fills the screen. Zoom out reveals the tree.
-- Pages default to A4, or auto-match the aspect ratio of an imported PDF page; resizable manually otherwise.
+- Pages default to A4, or auto-match the aspect ratio of an imported PDF page; resizable manually otherwise (page menu: presets, or a custom size in mm).
 
 ## 2. Flatten Algorithm (tree → linear page order, e.g. for PDF page order / print)
 
@@ -45,7 +46,7 @@ Canvases (PDF files) live in a **file tree** you define, e.g. `school/english/le
 
 ## 5. Sync & Storage
 
-- Repo-backed (GitHub/GitLab), one file per canvas.
+- Repo-backed (GitHub for now; GitLab not implemented), one file per canvas.
 - PWA: offline-first, caches locally, explicit save/sync pushes to repo.
 - Multi-device: git pull/push is the sync mechanism (not real-time collab).
 
@@ -54,7 +55,7 @@ Canvases (PDF files) live in a **file tree** you define, e.g. `school/english/le
 - **Frontend**: SvelteKit, deployed on your VPS, installable as PWA.
 - **Ink input**: Pointer Events + pressure (your existing artifact — pending).
 - **PDF read/write**: pdf.js + pdf-lib.
-- **Storage**: Git repo via GitHub/GitLab API (or local backend proxy holding the token).
+- **Storage**: IndexedDB locally; Git repo via the GitHub REST API (token in the browser, no proxy).
 
 ## 7. Confirmed Product Features
 
@@ -79,12 +80,23 @@ Source: user-provided `palm-rejection-test.html` test harness. The acceptance po
 - A `touch` pointer is rejected if any of: a pen stroke is active or was active within the last ~5s, its contact ellipse (`width`/`height`) exceeds ~35px (palm heel vs. fingertip), it's within 150ms of the last pen activity, or a pen is currently hovering (armed lockout before it even touches down).
 - A touch stroke already in progress gets cut off mid-stroke if pen activity appears.
 
-**Gaps before this is the real ink engine** (currently a raster test, not vector):
+**How the app applies it** (`src/lib/ink/palm.ts`):
-- `draw()` writes straight to canvas via `ctx.lineTo`/`stroke` with pressure-scaled line width, and throws points away. Needs to become `recordPoint()`: push `{x, y, pressure, tiltX, tiltY, t}` into a `Stroke.points[]` array per stroke; rendering = replaying that array. This is what makes strokes resizable/erasable/undoable and exportable to both the PDF Ink-annotation layer and the JSON attachment layer.
+- Each touch is evaluated with an intent. `draw` (finger ink) gets the full policy above. `navigate` (pan, pinch, and taps on canvas buttons such as the add-page "+") skips the 5 s pen session and the hover lockout, so fingers can move around and tap between pen strokes; pen-down, wide contacts and the 150 ms window still reject it.
-- `tiltX`/`tiltY` exist on the pen's `PointerEvent` but aren't read anywhere yet — needed for tilt support.
+- Once a pen has been used, fingers only navigate. "Draw with finger" in Settings turns finger ink off entirely.
-- No smoothing yet (raw straight segments) — fine as a v1, can layer in curve smoothing later without changing point storage.
+- Each rule (pen session, wide contact, timing, hover) can be switched off in Settings → Pen & touch.
-- No undo/redo, multi-color, eraser yet — needs a `Stroke[]` array per page's ink layer with add/remove, which undo/redo just pops/pushes.
+- An interrupted pointer (`pointercancel`) drops the stroke or erase in progress instead of committing it.
+
+**Ink engine (implemented)**:
+- `StrokeRecorder.recordPoint()` stores `x, y, pressure, tiltX, tiltY, t` per sample in a flat `Stroke.points` array (stride 6); rendering replays it. Tilt is recorded but not yet used for rendering.
+- Outlines are smoothed with `perfect-freehand`; mouse and finger strokes simulate pressure.
+- Pen, highlighter (translucent, no thinning) and stroke eraser, with configurable colour palettes; the pen's eraser end erases too.
+- Undo/redo covers ink, erasing and page-tree changes.
+- Strokes are written to the PDF both as Ink annotations and as JSON attachments.
 
 ## 10. Open Items
 
-- [ ] Nothing blocking — ready to move into implementation planning (component breakdown, page-tree data structures in Svelte, PDF encode/decode module)
+- [ ] Conflict copies: sync currently lets the newer version win silently; keep the losing version as a separate file.
+- [ ] Tell the user when a new version of the app has been deployed (the service worker updates, but the open tab keeps old code until reload).
+- [ ] CI that runs `check`, `test` and `build` on every pull request.
+- [ ] Use recorded tilt for rendering.
+- [ ] GitLab sync.

src/app.css

@@ -61,6 +61,7 @@
 	--shadow: 0 1px 2px rgba(0, 0, 0, 0.06), 0 4px 16px rgba(0, 0, 0, 0.08);
 	--page-line: rgba(0, 0, 0, 0.18);
 	--grid-line: rgba(0, 0, 0, 0.05);
+	--grid-line-minor: rgba(0, 0, 0, 0.028);
 	--ghost: rgba(0, 0, 0, 0.045);
 	--ghost-border: rgba(0, 0, 0, 0.12);
 	--radius: 6px;
@@ -88,6 +89,7 @@
 	--shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 6px 20px rgba(0, 0, 0, 0.35);
 	--page-line: rgba(255, 255, 255, 0.16);
 	--grid-line: rgba(255, 255, 255, 0.04);
+	--grid-line-minor: rgba(255, 255, 255, 0.022);
 	--ghost: rgba(255, 255, 255, 0.04);
 	--ghost-border: rgba(255, 255, 255, 0.14);
 	color-scheme: dark;

src/lib/components/CanvasView.svelte

@@ -16,10 +16,12 @@
 	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 '$lib/ink/palm';
+	import { flingDir, pushSample, type Sample } from '$lib/ink/fling';
 	import { StrokeRecorder, hitStroke } from '$lib/ink/stroke';
 	import { pageCssVars } from '$lib/editor/pageStyle';
 	import { downloadCanvas } from './download';
+	import PageSizeDialog from './PageSizeDialog.svelte';
 	import PageView from './PageView.svelte';
 	import Toolbar from './Toolbar.svelte';
 
@@ -28,11 +30,28 @@
 	let stage: HTMLDivElement;
 	const cssVars = pageCssVars();
 
-	// Background grid: one cell per active page, aligned to its edges (A4 at the origin if none).
+	// Background grid: one cell per active page, aligned to its edges (A4 at the origin if none),
-	const grid = $derived.by(() => {
+	// optionally split into fainter subdivisions.
+	const gridStyle = $derived.by(() => {
+		const g = settings.data.grid;
+		if (!g.show) return '';
 		const r = (doc.activeId && doc.rects.get(doc.activeId)) || { x: 0, y: 0, ...A4 };
 		const s = viewport.scale;
-		return { w: r.width * s, h: r.height * s, x: viewport.x + r.x * s, y: viewport.y + r.y * s };
+		const w = r.width * s;
+		const h = r.height * s;
+		const lines = (color: string) => [
+			`linear-gradient(to right, ${color} 1px, transparent 1px)`,
+			`linear-gradient(to bottom, ${color} 1px, transparent 1px)`
+		];
+		const images = lines('var(--grid-line)');
+		const sizes = [`${w}px ${h}px`, `${w}px ${h}px`];
+		// Leave out subdivisions once they'd be too dense to read.
+		const n = g.divisions;
+		if (n > 1 && Math.min(w, h) / n >= 8) {
+			images.push(...lines('var(--grid-line-minor)'));
+			sizes.push(`${w / n}px ${h / n}px`, `${w / n}px ${h / n}px`);
+		}
+		return `background-image: ${images.join(', ')}; background-size: ${sizes.join(', ')}; background-position: ${viewport.x + r.x * s}px ${viewport.y + r.y * s}px`;
 	});
 
 	// ---- autosave --------------------------------------------------------
@@ -55,6 +74,7 @@
 		return () => {
 			ro.disconnect();
 			stage.removeEventListener('wheel', onWheel);
+			clearTimeout(offerTimer);
 		};
 	});
 
@@ -99,6 +119,34 @@
 
 	const placeSlots = $derived(tools.placing ? freeSlots(doc.tree) : []);
 
+	// A finger fling toward a free side of the active page offers a new page there: the plus
+	// pulses if it's on screen, otherwise a button appears at that edge of the screen.
+	let offer = $state<Slot | null>(null);
+	let offerTimer: ReturnType<typeof setTimeout> | undefined;
+
+	function setOffer(slot: Slot | null) {
+		clearTimeout(offerTimer);
+		offer = slot;
+		if (slot) offerTimer = setTimeout(() => (offer = null), 3000);
+	}
+
+	function offerAfterFling(samples: Sample[]) {
+		if (!settings.data.flingToAdd) return;
+		const dir = flingDir(samples);
+		const id = doc.activeId;
+		const slot = dir && ghostSlots.find((s) => s.anchor === id && s.dir === dir);
+		if (slot) setOffer(slot);
+	}
+
+	/** The offered plus is off screen, so the offer shows as an edge-of-screen button instead. */
+	const offerOffscreen = $derived.by(() => {
+		const r = offer && plusRect(offer);
+		if (!r) return false;
+		const a = viewport.toScreen(r.x, r.y);
+		const b = viewport.toScreen(r.x + r.width, r.y + r.height);
+		return b.x < 0 || b.y < 0 || a.x > viewport.width || a.y > viewport.height;
+	});
+
 	// Show every possible spot when an imported PDF is waiting to be placed.
 	$effect(() => {
 		if (!tools.placing) return;
@@ -215,15 +263,9 @@
 		menu.show(x, y, items);
 	}
 
-	function customSize(id: string) {
+	/** Page whose custom-size dialog is open. */
-		const page = doc.pages[id];
+	let sizing = $state<string | null>(null);
-		const mm = (pt: number) => Math.round((pt / 72) * 25.4);
+	const customSize = (id: string) => (sizing = id);
-		const answer = prompt('Page size in millimetres (width × height)', `${mm(page.width)} × ${mm(page.height)}`);
-		const m = answer && /^\s*(\d+(?:\.\d+)?)\s*[x×*,\s]\s*(\d+(?:\.\d+)?)\s*$/i.exec(answer);
-		if (!m) return;
-		const pt = (v: string) => Math.min(5000, Math.max(50, (parseFloat(v) / 25.4) * 72));
-		keepAnchor(id, () => doc.resizePage(id, pt(m[1]), pt(m[2])));
-	}
 
 	const activeMenuPos = $derived.by(() => {
 		const id = doc.activeId;
@@ -242,7 +284,7 @@
 	type Gesture =
 		| { kind: 'draw'; pointerId: number; pointerType: string; pageId: string; rec: StrokeRecorder }
 		| { kind: 'erase'; pointerId: number; pointerType: string; hits: Map<string, Set<string>> }
-		| { kind: 'pan'; pointerId: number; lastX: number; lastY: number; startX: number; startY: number; moved: boolean; tap: boolean }
+		| { kind: 'pan'; pointerId: number; pointerType: string; lastX: number; lastY: number; startX: number; startY: number; moved: boolean; tap: boolean; trail: Sample[] }
 		| { kind: 'add'; pointerId: number; pointerType: string; startX: number; startY: number; slot: Slot }
 		| { kind: 'pinch' };
 
@@ -291,6 +333,7 @@
 	function onPointerDown(e: PointerEvent) {
 		palm.track(e);
 		if (menu.open) menu.close();
+		if (offer) setOffer(null);
 		const target = e.target as Element;
 		if (target.closest('.no-stage')) return;
 		const p = local(e);
@@ -299,9 +342,9 @@
 		const fingerInks = tools.inking && settings.data.fingerDraw && !palm.penSeen;
 		// Taps on an add-page button are palm-checked like navigation: still rejected for
 		// pen-down, palm-sized contacts and right after pen use, but not for the 5 s pen session.
-		const plus = target.closest<HTMLElement>('.ghost');
+		const plus = target.closest<HTMLElement>('.ghost, .offer-pill');
 		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) {
@@ -334,7 +377,18 @@
 			(e.pointerType === 'mouse' && e.button === 0 && !onPage && !target.closest('.slot'));
 		if (wantsPan) {
 			if (onPage && !inEditor) doc.activeId = onPage;
-			gesture = { kind: 'pan', pointerId: e.pointerId, lastX: p.x, lastY: p.y, startX: p.x, startY: p.y, moved: false, tap: inEditor || !!target.closest('.slot') };
+			gesture = {
+				kind: 'pan',
+				pointerId: e.pointerId,
+				pointerType: e.pointerType,
+				lastX: p.x,
+				lastY: p.y,
+				startX: p.x,
+				startY: p.y,
+				moved: false,
+				tap: inEditor || !!target.closest('.slot'),
+				trail: [{ ...p, t: e.timeStamp }]
+			};
 			if (e.pointerType !== 'touch' || !gesture.tap) {
 				if (e.pointerType !== 'touch') e.preventDefault();
 				capture(e.pointerId);
@@ -393,6 +447,7 @@
 			viewport.panBy(dx, dy);
 			gesture.lastX = p.x;
 			gesture.lastY = p.y;
+			pushSample(gesture.trail, { ...p, t: e.timeStamp });
 			return;
 		}
 
@@ -436,13 +491,19 @@
 		if (!gesture || gesture.pointerId !== e.pointerId) return;
 		const g = gesture;
 		gesture = null;
-		if (g.kind === 'add') {
+		if (g.kind === 'pan') {
+			if (e.type === 'pointerup' && g.moved && g.pointerType === 'touch') {
+				pushSample(g.trail, { ...local(e), t: e.timeStamp });
+				offerAfterFling(g.trail);
+			}
+		} else if (g.kind === 'add') {
 			if (e.type === 'pointerup' && (g.pointerType !== 'touch' || palm.evaluate(e, 'navigate').accept)) addPage(g.slot.anchor, g.slot.dir);
 		} else if (g.kind === 'draw') {
 			live = null;
 			if (e.type === 'pointerup') doc.addStroke(g.pageId, g.rec.stroke);
 		} else if (g.kind === 'erase') {
-			for (const [pageId, ids] of g.hits) doc.setStrokes(pageId, (s) => s.filter((x) => !ids.has(x.id)));
+			// Like ink, an interrupted (pointercancel) erase is dropped.
+			if (e.type === 'pointerup') for (const [pageId, ids] of g.hits) doc.setStrokes(pageId, (s) => s.filter((x) => !ids.has(x.id)));
 			erasing = new Set();
 		}
 	}
@@ -506,7 +567,7 @@
 	}
 
 	function onKeyDown(e: KeyboardEvent) {
-		if (workspace.settingsOpen) return;
+		if (workspace.settingsOpen || sizing) return;
 		const mod = e.ctrlKey || e.metaKey;
 		const k = e.key.toLowerCase();
 
@@ -565,7 +626,7 @@
 		class:panning
 		class:erasing={tools.tool === 'eraser'}
 		bind:this={stage}
-		style="background-size: {grid.w}px {grid.h}px; background-position: {grid.x}px {grid.y}px"
+		style={gridStyle}
 		onpointerdown={onPointerDown}
 		onpointermove={onPointerMove}
 		onpointerup={onPointerUp}
@@ -605,6 +666,7 @@
 					<!-- Pointer taps go through the palm-checked gesture; onclick only serves the keyboard. -->
 					<button
 						class="ghost"
+						class:offer={offer?.anchor === slot.anchor && offer.dir === slot.dir}
 						data-anchor={slot.anchor}
 						data-dir={slot.dir}
 						style="left:{r.x}px; top:{r.y}px; width:{r.width}px; height:{r.height}px"
@@ -649,6 +711,20 @@
 			</button>
 		{/if}
 
+		{#if offer && offerOffscreen}
+			<!-- Like the plus buttons, taps go through the palm-checked gesture. -->
+			<button
+				class="offer-pill"
+				data-anchor={offer.anchor}
+				data-dir={offer.dir}
+				data-side={offer.dir}
+				onclick={(e) => e.detail === 0 && offer && addPage(offer.anchor, offer.dir)}
+			>
+				<CirclePlusIcon size={18} strokeWidth={1.75} />
+				Add page
+			</button>
+		{/if}
+
 		{#if eraserAt && tools.tool === 'eraser'}
 			<div class="eraser-ring" style="left:{eraserAt.x}px; top:{eraserAt.y}px; width:{tools.eraserRadius * 2}px; height:{tools.eraserRadius * 2}px"></div>
 		{/if}
@@ -661,6 +737,19 @@
 		{/if}
 	</div>
 
+	{#if sizing && doc.pages[sizing]}
+		{@const id = sizing}
+		<PageSizeDialog
+			width={doc.pages[id].width}
+			height={doc.pages[id].height}
+			onclose={() => (sizing = null)}
+			onapply={(w, h) => {
+				sizing = null;
+				keepAnchor(id, () => doc.resizePage(id, w, h));
+			}}
+		/>
+	{/if}
+
 	<Toolbar {doc} onfit={() => fitPage(doc.activeId)} onfitall={fitAll} />
 </div>
 
@@ -706,9 +795,6 @@
 		user-select: none;
 		-webkit-user-select: none;
 		background-color: var(--bg-canvas);
-		background-image:
-			linear-gradient(to right, var(--grid-line) 1px, transparent 1px),
-			linear-gradient(to bottom, var(--grid-line) 1px, transparent 1px);
 	}
 	.stage :global(.cm-content) {
 		user-select: text;
@@ -755,6 +841,57 @@
 		border-radius: 50%;
 		background: var(--bg-canvas);
 	}
+	.ghost.offer {
+		color: var(--accent);
+	}
+	.ghost.offer .plus {
+		animation: offer 1s ease-in-out infinite;
+	}
+	@keyframes offer {
+		50% {
+			transform: scale(1.3);
+		}
+	}
+	.offer-pill {
+		position: absolute;
+		display: flex;
+		align-items: center;
+		gap: 6px;
+		padding: 10px 16px 10px 12px;
+		border: 1px solid var(--accent);
+		border-radius: 999px;
+		background: var(--bg);
+		color: var(--accent);
+		box-shadow: var(--shadow);
+		cursor: pointer;
+		white-space: nowrap;
+	}
+	.offer-pill[data-side='right'] {
+		right: 12px;
+		top: 50%;
+		transform: translateY(-50%);
+	}
+	.offer-pill[data-side='left'] {
+		left: 12px;
+		top: 50%;
+		transform: translateY(-50%);
+	}
+	.offer-pill[data-side='up'] {
+		top: 12px;
+		left: 50%;
+		transform: translateX(-50%);
+	}
+	.offer-pill[data-side='down'] {
+		/* Clear of the toolbar. */
+		bottom: 72px;
+		left: 50%;
+		transform: translateX(-50%);
+	}
+	@media (prefers-reduced-motion: reduce) {
+		.ghost.offer .plus {
+			animation: none;
+		}
+	}
 	.slot {
 		position: absolute;
 		display: grid;

src/lib/components/PageSizeDialog.svelte

@@ -0,0 +1,173 @@
+<script lang="ts">
+	import { untrack } from 'svelte';
+	import ArrowLeftRightIcon from '@lucide/svelte/icons/arrow-left-right';
+	import { PAGE_PRESETS } from '$lib/model/types';
+
+	let {
+		width,
+		height,
+		onapply,
+		onclose
+	}: { width: number; height: number; onapply: (width: number, height: number) => void; onclose: () => void } = $props();
+
+	// Sizes are stored in points; people think in millimetres.
+	const MIN_PT = 50;
+	const MAX_PT = 5000;
+	const toMm = (pt: number) => Math.round((pt / 72) * 25.4);
+	const toPt = (mm: number) => (mm / 25.4) * 72;
+
+	// Start from the page's current size; edits are local until Apply.
+	let w = $state(untrack(() => toMm(width)));
+	let h = $state(untrack(() => toMm(height)));
+	let first: HTMLInputElement;
+
+	const valid = $derived([w, h].every((v) => Number.isFinite(v) && toPt(v) >= MIN_PT && toPt(v) <= MAX_PT));
+
+	$effect(() => {
+		first.focus();
+		first.select();
+	});
+
+	function apply() {
+		if (!valid) return;
+		// Whole millimetres would nudge exact sizes (A4 is 595.28 pt), so keep those as they are.
+		const exact = [{ width, height }, ...PAGE_PRESETS].find((p) => toMm(p.width) === w && toMm(p.height) === h);
+		if (exact) onapply(exact.width, exact.height);
+		else onapply(toPt(w), toPt(h));
+	}
+</script>
+
+<svelte:window onkeydown={(e) => e.key === 'Escape' && onclose()} />
+
+<!-- svelte-ignore a11y_click_events_have_key_events, a11y_no_static_element_interactions -->
+<div class="backdrop" onclick={(e) => e.target === e.currentTarget && onclose()}>
+	<form
+		class="dialog"
+		aria-label="Page size"
+		onsubmit={(e) => {
+			e.preventDefault();
+			apply();
+		}}
+	>
+		<h2>Page size</h2>
+		<div class="presets">
+			{#each PAGE_PRESETS as p (p.label)}
+				<button
+					type="button"
+					class="chip"
+					class:active={toMm(p.width) === w && toMm(p.height) === h}
+					onclick={() => ((w = toMm(p.width)), (h = toMm(p.height)))}>{p.label}</button
+				>
+			{/each}
+		</div>
+		<div class="fields">
+			<label>
+				<span>Width</span>
+				<input type="number" inputmode="decimal" min={toMm(MIN_PT)} max={toMm(MAX_PT)} step="1" bind:value={w} bind:this={first} />
+			</label>
+			<button type="button" class="icon-btn" title="Swap width and height" onclick={() => ([w, h] = [h, w])}>
+				<ArrowLeftRightIcon size={16} />
+			</button>
+			<label>
+				<span>Height</span>
+				<input type="number" inputmode="decimal" min={toMm(MIN_PT)} max={toMm(MAX_PT)} step="1" bind:value={h} />
+			</label>
+			<span class="unit">mm</span>
+		</div>
+		{#if !valid}
+			<p class="hint">Sizes must be between {toMm(MIN_PT)} and {toMm(MAX_PT)} mm.</p>
+		{/if}
+		<div class="actions">
+			<button type="button" class="btn" onclick={onclose}>Cancel</button>
+			<button type="submit" class="btn primary" disabled={!valid}>Apply</button>
+		</div>
+	</form>
+</div>
+
+<style>
+	.backdrop {
+		position: fixed;
+		inset: 0;
+		z-index: 50;
+		display: grid;
+		place-items: center;
+		background: rgba(0, 0, 0, 0.45);
+		padding: 16px;
+	}
+	.dialog {
+		width: min(400px, 100%);
+		background: var(--bg);
+		border: 1px solid var(--border);
+		border-radius: 10px;
+		box-shadow: var(--shadow);
+		padding: 18px 20px 20px;
+	}
+	h2 {
+		font-size: 16px;
+		margin: 0 0 14px;
+	}
+	.presets {
+		display: flex;
+		flex-wrap: wrap;
+		gap: 6px;
+		margin-bottom: 16px;
+	}
+	.chip {
+		border: 1px solid var(--border);
+		background: none;
+		color: var(--muted);
+		border-radius: 14px;
+		padding: 4px 10px;
+		cursor: pointer;
+	}
+	.chip:hover {
+		color: var(--text);
+		background: var(--hover);
+	}
+	.chip.active {
+		color: var(--text);
+		border-color: var(--accent);
+		background: var(--accent-soft);
+	}
+	.fields {
+		display: flex;
+		align-items: flex-end;
+		gap: 8px;
+	}
+	label {
+		display: flex;
+		flex-direction: column;
+		gap: 4px;
+		flex: 1;
+		min-width: 0;
+	}
+	label span {
+		font-size: 12px;
+		color: var(--muted);
+	}
+	input {
+		width: 100%;
+	}
+	.fields .icon-btn {
+		margin-bottom: 2px;
+	}
+	.unit {
+		color: var(--faint);
+		padding-bottom: 7px;
+	}
+	.hint {
+		color: var(--danger);
+		font-size: 12px;
+		margin: 8px 0 0;
+	}
+	.actions {
+		display: flex;
+		justify-content: flex-end;
+		gap: 8px;
+		margin-top: 20px;
+	}
+	.btn:disabled {
+		opacity: 0.5;
+		cursor: default;
+	}
+</style>

src/lib/components/SettingsModal.svelte

@@ -83,12 +83,30 @@
 						<option value="paper">Paper</option>
 					</select>
 				</div>
+				<div class="row">
+					<div><div class="name">Background grid</div><div class="desc">Faint lines behind the pages, one cell per active page.</div></div>
+					<input type="checkbox" class="toggle" bind:checked={s.grid.show} onchange={save} />
+				</div>
+				<div class="row">
+					<div><div class="name">Grid subdivisions</div><div class="desc">Split each page-sized cell into a finer grid for sketching and lining things up.</div></div>
+					<select bind:value={s.grid.divisions} onchange={save} disabled={!s.grid.show}>
+						<option value={1}>None</option>
+						<option value={2}>2 × 2</option>
+						<option value={4}>4 × 4</option>
+						<option value={8}>8 × 8</option>
+						<option value={16}>16 × 16</option>
+					</select>
+				</div>
 			{:else if tab === 'input'}
 				<h2>Pen &amp; touch</h2>
 				<div class="row">
 					<div><div class="name">Draw with finger</div><div class="desc">When off, a single finger always pans; only pen and mouse draw. Once you use a pen, fingers only pan and zoom.</div></div>
 					<input type="checkbox" class="toggle" bind:checked={s.fingerDraw} onchange={save} />
 				</div>
+				<div class="row">
+					<div><div class="name">Swipe to add pages</div><div class="desc">A quick one-finger flick toward a free side of the active page offers to add a page there.</div></div>
+					<input type="checkbox" class="toggle" bind:checked={s.flingToAdd} onchange={save} />
+				</div>
 				<h3>Palm rejection</h3>
 				<div class="row">
 					<div><div class="name">Pen session lockout</div><div class="desc">Ignore touches while the pen is writing, and finger ink for 5 seconds after it was used.</div></div>

src/lib/ink/fling.test.ts

@@ -0,0 +1,43 @@
+import { describe, expect, it } from 'vitest';
+import { flingDir, pushSample, type Sample } from './fling';
+
+/** A straight drag from (0,0) by (dx,dy) over `ms`, sampled every 16 ms. */
+function drag(dx: number, dy: number, ms: number): Sample[] {
+	const out: Sample[] = [];
+	for (let t = 0; t <= ms; t += 16) pushSample(out, { x: (dx * t) / ms, y: (dy * t) / ms, t });
+	return out;
+}
+
+describe('flingDir', () => {
+	it('maps a flick to the side it reveals', () => {
+		expect(flingDir(drag(-200, 0, 96))).toBe('right');
+		expect(flingDir(drag(200, 0, 96))).toBe('left');
+		expect(flingDir(drag(0, -200, 96))).toBe('down');
+		expect(flingDir(drag(0, 200, 96))).toBe('up');
+	});
+
+	it('ignores slow drags', () => {
+		expect(flingDir(drag(-200, 0, 800))).toBeNull();
+	});
+
+	it('ignores diagonal flicks', () => {
+		expect(flingDir(drag(-200, -150, 96))).toBeNull();
+	});
+
+	it('ignores a drag that stops before lifting', () => {
+		const s = drag(-300, 0, 96);
+		const end = s.at(-1)!;
+		for (let t = 16; t <= 160; t += 16) pushSample(s, { ...end, t: end.t + t });
+		expect(flingDir(s)).toBeNull();
+	});
+
+	it('needs at least two samples', () => {
+		expect(flingDir([])).toBeNull();
+		expect(flingDir([{ x: 0, y: 0, t: 0 }])).toBeNull();
+	});
+
+	it('keeps the trail short', () => {
+		const s = drag(-2000, 0, 2000);
+		expect(s.at(-1)!.t - s[0].t).toBeLessThanOrEqual(216);
+	});
+});

src/lib/ink/fling.ts

@@ -0,0 +1,41 @@
+// Fling detection for one-finger pans: a quick flick that ends the drag.
+
+import type { Dir } from '$lib/model/tree';
+
+export interface Sample {
+	x: number;
+	y: number;
+	t: number;
+}
+
+/** Only the last stretch of the drag counts. */
+const WINDOW_MS = 100;
+/** px/ms; a slow drag that stops is not a fling. */
+const MIN_SPEED = 0.6;
+/** The main axis must dominate by this much. */
+const MIN_RATIO = 2;
+
+/**
+ * Side of the canvas a fling heads for, or null. Moving the finger left pulls in
+ * what's on the right, so a leftward flick returns 'right'.
+ */
+export function flingDir(samples: readonly Sample[]): Dir | null {
+	const last = samples.at(-1);
+	if (!last) return null;
+	const first = samples.find((s) => last.t - s.t <= WINDOW_MS);
+	if (!first || first === last) return null;
+	const dt = last.t - first.t;
+	if (dt <= 0) return null;
+	const dx = last.x - first.x;
+	const dy = last.y - first.y;
+	if (Math.hypot(dx, dy) / dt < MIN_SPEED) return null;
+	if (Math.abs(dx) >= Math.abs(dy) * MIN_RATIO) return dx < 0 ? 'right' : 'left';
+	if (Math.abs(dy) >= Math.abs(dx) * MIN_RATIO) return dy < 0 ? 'down' : 'up';
+	return null;
+}
+
+/** Keeps a short trail of pointer samples for flingDir. */
+export function pushSample(samples: Sample[], s: Sample) {
+	samples.push(s);
+	while (samples.length > 2 && s.t - samples[0].t > WINDOW_MS * 2) samples.shift();
+}

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;

src/lib/state/settings.svelte.ts

@@ -6,6 +6,12 @@ import { DEFAULT_PALM, type PalmOptions } from '$lib/ink/palm';
 export type ThemePref = 'system' | 'light' | 'dark';
 export type PageStylePref = 'paper' | 'match';
 
+export interface GridSettings {
+	show: boolean;
+	/** Minor lines per page cell side (1 = page-sized cells only). */
+	divisions: number;
+}
+
 export interface GitHubSettings {
 	token: string;
 	owner: string;
@@ -18,7 +24,10 @@ export interface GitHubSettings {
 interface SettingsData {
 	theme: ThemePref;
 	pageStyle: PageStylePref;
+	grid: GridSettings;
 	fingerDraw: boolean;
+	/** A finger fling toward a free side of the active page offers a new page there. */
+	flingToAdd: boolean;
 	palm: PalmOptions;
 	github: GitHubSettings;
 	/** Minutes between automatic pushes (0 = manual only). */
@@ -30,7 +39,9 @@ interface SettingsData {
 const DEFAULTS: SettingsData = {
 	theme: 'system',
 	pageStyle: 'match',
+	grid: { show: true, divisions: 1 },
 	fingerDraw: true,
+	flingToAdd: true,
 	palm: DEFAULT_PALM,
 	github: { token: '', owner: '', repo: '', branch: 'main', dir: '' },
 	pushInterval: 5,
@@ -48,6 +59,7 @@ function load(): SettingsData {
 		return {
 			...DEFAULTS,
 			...saved,
+			grid: { ...DEFAULTS.grid, ...saved.grid },
 			palm: { ...DEFAULTS.palm, ...saved.palm },
 			github: { ...DEFAULTS.github, ...saved.github }
 		};