diff --git a/web/src/lib/components/ui/Tooltip.stories.svelte b/web/src/lib/components/ui/Tooltip.stories.svelte
new file mode 100644
index 00000000..fb43523c
--- /dev/null
+++ b/web/src/lib/components/ui/Tooltip.stories.svelte
@@ -0,0 +1,94 @@
+<script module lang="ts">
+	import { defineMeta } from "@storybook/addon-svelte-csf";
+	import Plus from "$icon/plus";
+	import Button from "./Button.svelte";
+	import Tooltip from "./Tooltip.svelte";
+
+	const { Story } = defineMeta({
+		title: "UI/Tooltip",
+		component: Tooltip,
+		tags: ["autodocs"]
+	});
+</script>
+
+<!-- Tooltip's `children` is the trigger it wraps, so every story renders via `asChild`
+	rather than through the args table. Hover or tab to the trigger in the canvas — `open`
+	is internal state unless you bind it. The stories pad generously because the bubble is
+	a popover in the top layer and would otherwise sit over the canvas edge. -->
+
+{#snippet richContent()}
+	<span class="font-medium">Sign the commit</span>
+	<span class="block text-foreground-on-emphasis/70">
+		Requires a key registered on your account.
+	</span>
+{/snippet}
+
+<Story name="Default" asChild>
+	<div class="flex justify-center p-16">
+		<Tooltip content="Add to library">
+			<Button>Hover me</Button>
+		</Tooltip>
+	</div>
+</Story>
+
+<!-- `side` picks the preferred edge; the bubble still flips to the opposite one when the
+	preferred side would overflow the viewport. -->
+<Story name="Sides" asChild>
+	<div class="flex justify-center gap-2 p-16">
+		{#each ["left", "top", "bottom", "right"] as const as side (side)}
+			<Tooltip {side} content={side}>
+				<Button>{side}</Button>
+			</Tooltip>
+		{/each}
+	</div>
+</Story>
+
+<!-- the canonical use: an icon-only button whose aria-label and tooltip say the same thing -->
+<Story name="Icon button" asChild>
+	<div class="flex justify-center p-16">
+		<Tooltip content="New issue">
+			<Button icon={Plus} aria-label="New issue" class="px-2" />
+		</Tooltip>
+	</div>
+</Story>
+
+<!-- `content` also takes a snippet when a single string is not enough -->
+<Story name="Rich content" asChild>
+	<div class="flex justify-center p-16">
+		<Tooltip content={richContent}>
+			<Button>Signed commits</Button>
+		</Tooltip>
+	</div>
+</Story>
+
+<!-- `delay={0}` opens on hover with no wait. Note that once any tooltip has been shown,
+	moving to a neighbouring trigger opens instantly regardless of delay. -->
+<Story name="No delay" asChild>
+	<div class="flex justify-center gap-2 p-16">
+		<Tooltip content="Opens immediately" delay={0}>
+			<Button>No delay</Button>
+		</Tooltip>
+		<Tooltip content="Waits 700ms" delay={700}>
+			<Button>Slow</Button>
+		</Tooltip>
+	</div>
+</Story>
+
+<!-- the arrow points at whichever edge the bubble actually landed on, so it stays correct
+	even when the preferred side had to flip. `arrow={false}` drops it. -->
+<Story name="No arrow" asChild>
+	<div class="flex justify-center p-16">
+		<Tooltip content="No pointer" arrow={false}>
+			<Button>No arrow</Button>
+		</Tooltip>
+	</div>
+</Story>
+
+<!-- `disabled` skips the tooltip without changing the markup around the trigger -->
+<Story name="Disabled" asChild>
+	<div class="flex justify-center p-16">
+		<Tooltip content="You will not see this" disabled>
+			<Button>No tooltip</Button>
+		</Tooltip>
+	</div>
+</Story>
diff --git a/web/src/lib/components/ui/Tooltip.svelte b/web/src/lib/components/ui/Tooltip.svelte
new file mode 100644
index 00000000..743e440a
--- /dev/null
+++ b/web/src/lib/components/ui/Tooltip.svelte
@@ -0,0 +1,367 @@
+<script module lang="ts">
+	import { tv, type VariantProps } from "tailwind-variants";
+
+	export const tooltip = tv({
+		slots: {
+			// the wrapper is the anchor: it has to hug its child so the tooltip lines up with
+			// the trigger and not with a full-width box around it
+			wrapper: "inline-flex",
+			// overflow-visible undoes the UA stylesheet's `overflow: auto` on [popover], which
+			// would otherwise clip the arrow off at the bubble's padding box
+			bubble:
+				"tooltip-bubble fixed z-50 m-0 w-max max-w-64 overflow-visible rounded-sm bg-background-emphasis px-2 pt-1.5 pb-1 typography-paragraph-small text-foreground-inverted shadow-regular"
+		},
+		variants: {
+			side: {
+				top: {},
+				right: {},
+				bottom: {},
+				left: {}
+			}
+		},
+		defaultVariants: {
+			side: "top"
+		}
+	});
+
+	export type TooltipVariants = VariantProps<typeof tooltip>;
+
+	// hovering a second trigger while one tooltip is up should swap them, not stack them.
+	// `popover="hint"` would do this natively but is not widely supported yet, so keep a
+	// module-level singleton the way Dropdown keeps its groups.
+	let openTooltip: (() => void) | undefined;
+
+	// once a tooltip has been shown, moving to a neighbouring trigger inside this window
+	// opens instantly — a toolbar of icon buttons feels broken if every hop re-waits.
+	const SKIP_DELAY = 300;
+	let lastHiddenAt = 0;
+</script>
+
+<script lang="ts">
+	import type { Snippet } from "svelte";
+	import { onMount } from "svelte";
+
+	interface Props {
+		/** Tooltip body: a plain string for the common case, a snippet for rich content. */
+		content: string | Snippet;
+		side?: TooltipVariants["side"];
+		/** ms to wait before opening on hover. Keyboard focus always opens immediately. */
+		delay?: number;
+		/** Skip the tooltip entirely — handy when the label is only useful in some states. */
+		disabled?: boolean;
+		/** Two-way: set it to drive the tooltip yourself, e.g. a transient "Copied!". */
+		open?: boolean;
+		/** Draw the little pointer at the trigger. */
+		arrow?: boolean;
+		id?: string;
+		class?: string;
+		bubbleClass?: string;
+		children: Snippet;
+	}
+
+	let {
+		content,
+		side = "top",
+		delay = 300,
+		disabled = false,
+		open = $bindable(false),
+		arrow = true,
+		id,
+		class: className,
+		bubbleClass,
+		children
+	}: Props = $props();
+
+	const classes = $derived(tooltip({ side }));
+	const fallbackId = $props.id();
+	const bubbleId = $derived(id ?? fallbackId);
+	// anchor-name has to be a custom ident and unique per instance, so it cannot live in the
+	// stylesheet — derive it from the same id the bubble is labelled with
+	const anchorName = $derived(`--tooltip-${bubbleId.replace(/[^a-zA-Z0-9_-]/g, "-")}`);
+
+	let wrapperEl = $state<HTMLElement>();
+	let bubbleEl = $state<HTMLElement>();
+	let timer: ReturnType<typeof setTimeout> | undefined;
+
+	// The arrow cannot be driven off `side`: position-try flips the bubble to the opposite
+	// edge without touching any attribute, so a preference of "top" can render below the
+	// trigger and the arrow would point away from it. Both the edge the bubble actually
+	// landed on and the arrow's offset along that edge are therefore measured from geometry.
+	// undefined until the first measurement, so `side` stays the honest answer until then
+	let measuredSide = $state<TooltipVariants["side"] | undefined>();
+	const resolvedSide = $derived(measuredSide ?? side);
+	let arrowOffset = $state(0);
+
+	// keeps the arrow clear of the bubble's rounded corners when the trigger sits near one end
+	const ARROW_INSET = 8;
+
+	const measureArrow = () => {
+		if (!arrow || !bubbleEl || !wrapperEl) return;
+		const bubble = bubbleEl.getBoundingClientRect();
+		const trigger = wrapperEl.getBoundingClientRect();
+		if (bubble.width === 0) return;
+
+		// only the axis the preferred side lives on can flip, so one comparison settles it
+		const vertical = side === "top" || side === "bottom";
+		measuredSide = vertical
+			? bubble.top >= trigger.bottom - 1
+				? "bottom"
+				: "top"
+			: bubble.left >= trigger.right - 1
+				? "right"
+				: "left";
+
+		// centre the arrow on the trigger rather than on the bubble: near a viewport edge the
+		// bubble slides sideways to stay visible while the trigger stays put
+		const along = vertical
+			? trigger.left + trigger.width / 2 - bubble.left
+			: trigger.top + trigger.height / 2 - bubble.top;
+		const extent = vertical ? bubble.width : bubble.height;
+		arrowOffset = Math.min(
+			Math.max(along, ARROW_INSET),
+			Math.max(extent - ARROW_INSET, ARROW_INSET)
+		);
+	};
+
+	const cancel = () => {
+		clearTimeout(timer);
+		timer = undefined;
+	};
+
+	const show = () => {
+		if (disabled) return;
+		cancel();
+		if (openTooltip && openTooltip !== hide) openTooltip();
+		openTooltip = hide;
+		bubbleEl?.showPopover();
+	};
+
+	const hide = () => {
+		cancel();
+		if (openTooltip === hide) openTooltip = undefined;
+		bubbleEl?.hidePopover();
+	};
+
+	const scheduleShow = () => {
+		if (disabled || open) return;
+		const instant = delay === 0 || Date.now() - lastHiddenAt < SKIP_DELAY;
+		if (instant) {
+			show();
+			return;
+		}
+		cancel();
+		timer = setTimeout(show, delay);
+	};
+
+	const onPointerEnter = (event: PointerEvent) => {
+		// touch has no hover: the tap would open the tooltip and swallow the visual feedback
+		// of whatever was actually tapped
+		if (event.pointerType === "touch") return;
+		scheduleShow();
+	};
+
+	const onFocusIn = () => {
+		// no delay on focus: aria-describedby only resolves while the bubble is rendered, so
+		// it has to be up by the time a keyboard user lands on the trigger
+		if (!disabled) show();
+	};
+
+	const onToggle = (event: Event) => {
+		const { newState } = event as ToggleEvent;
+		open = newState === "open";
+		if (!open) lastHiddenAt = Date.now();
+	};
+
+	$effect(() => {
+		if (!bubbleEl) return;
+		// keeps a consumer-driven `open` in sync with the popover; both calls are no-ops when
+		// the state already matches, so this cannot ping-pong with onToggle
+		if (open) bubbleEl.showPopover();
+		else bubbleEl.hidePopover();
+	});
+
+	$effect(() => {
+		if (disabled) hide();
+	});
+
+	// declared after the sync effect above so the popover is already rendered by the time this
+	// one measures — effects in the same flush run in declaration order
+	$effect(() => {
+		if (!open) return;
+		measureArrow();
+
+		const onKeydown = (event: KeyboardEvent) => {
+			if (event.key === "Escape") hide();
+		};
+		// the bubble tracks its anchor on scroll for free, but a flip mid-scroll would leave the
+		// arrow on the wrong edge, so re-measure whenever the geometry can have changed
+		document.addEventListener("keydown", onKeydown);
+		document.addEventListener("scroll", measureArrow, { capture: true, passive: true });
+		window.addEventListener("resize", measureArrow);
+		return () => {
+			document.removeEventListener("keydown", onKeydown);
+			document.removeEventListener("scroll", measureArrow, { capture: true });
+			window.removeEventListener("resize", measureArrow);
+		};
+	});
+
+	onMount(() => {
+		const wrapper = wrapperEl;
+		if (!wrapper) return;
+
+		// the handlers go on the wrapper rather than in the markup because a span carrying
+		// pointer handlers has no honest ARIA role to give the a11y lint — the interactive
+		// element is the consumer's child. pointerenter/leave don't bubble but do fire on the
+		// wrapper's own boundary, which encloses the trigger, so this is equivalent.
+		wrapper.addEventListener("pointerenter", onPointerEnter);
+		wrapper.addEventListener("pointerleave", hide);
+		wrapper.addEventListener("focusin", onFocusIn);
+		wrapper.addEventListener("focusout", hide);
+		// a click means the user acted; the label has served its purpose and is now in the way
+		wrapper.addEventListener("click", hide);
+
+		// point aria-describedby at whatever interactive element is inside rather than at the
+		// wrapper span, which AT ignores
+		const target =
+			wrapper.querySelector<HTMLElement>(
+				'button, a[href], input, select, textarea, [tabindex]:not([tabindex="-1"])'
+			) ?? wrapper;
+		const previous = target.getAttribute("aria-describedby");
+		target.setAttribute("aria-describedby", previous ? `${previous} ${bubbleId}` : bubbleId);
+
+		return () => {
+			wrapper.removeEventListener("pointerenter", onPointerEnter);
+			wrapper.removeEventListener("pointerleave", hide);
+			wrapper.removeEventListener("focusin", onFocusIn);
+			wrapper.removeEventListener("focusout", hide);
+			wrapper.removeEventListener("click", hide);
+			if (previous) target.setAttribute("aria-describedby", previous);
+			else target.removeAttribute("aria-describedby");
+			cancel();
+			if (openTooltip === hide) openTooltip = undefined;
+		};
+	});
+</script>
+
+<span
+	bind:this={wrapperEl}
+	class={classes.wrapper({ class: className })}
+	style="anchor-name: {anchorName}"
+>
+	{@render children()}
+</span>
+
+<div
+	bind:this={bubbleEl}
+	id={bubbleId}
+	role="tooltip"
+	popover="manual"
+	data-side={side}
+	data-resolved-side={resolvedSide}
+	data-arrow={arrow || undefined}
+	style="position-anchor: {anchorName}; --tooltip-arrow: {arrowOffset}px"
+	class={classes.bubble({ class: bubbleClass })}
+	ontoggle={onToggle}
+>
+	{#if typeof content === "string"}
+		{content}
+	{:else}
+		{@render content()}
+	{/if}
+</div>
+
+<style>
+	/* anchor positioning went Baseline in early 2026; without it the popover falls back to
+	   the UA's centred position, same trade-off Dropdown already takes */
+	@supports (position-area: bottom) and (position-try: flip-block) {
+		.tooltip-bubble {
+			/* the gap between trigger and bubble — margin, because position-area owns inset */
+			margin: 0.25rem;
+			position-try: flip-block, flip-inline;
+		}
+
+		.tooltip-bubble[data-side="top"] {
+			position-area: block-start;
+		}
+
+		.tooltip-bubble[data-side="bottom"] {
+			position-area: block-end;
+		}
+
+		.tooltip-bubble[data-side="left"] {
+			position-area: inline-start;
+		}
+
+		.tooltip-bubble[data-side="right"] {
+			position-area: inline-end;
+		}
+	}
+
+	/* the arrow exactly fills the 0.25rem gap, so its tip meets the trigger's edge.
+	   background-color: inherit keeps it in step with a bubbleClass that repaints the bubble. */
+	.tooltip-bubble[data-arrow]::after {
+		content: "";
+		position: absolute;
+		background-color: inherit;
+	}
+
+	.tooltip-bubble[data-arrow][data-resolved-side="top"]::after {
+		top: 100%;
+		left: var(--tooltip-arrow);
+		width: 0.5rem;
+		height: 0.25rem;
+		translate: -50% 0;
+		clip-path: polygon(50% 100%, 0 0, 100% 0);
+	}
+
+	.tooltip-bubble[data-arrow][data-resolved-side="bottom"]::after {
+		bottom: 100%;
+		left: var(--tooltip-arrow);
+		width: 0.5rem;
+		height: 0.25rem;
+		translate: -50% 0;
+		clip-path: polygon(50% 0, 0 100%, 100% 100%);
+	}
+
+	.tooltip-bubble[data-arrow][data-resolved-side="left"]::after {
+		left: 100%;
+		top: var(--tooltip-arrow);
+		width: 0.25rem;
+		height: 0.5rem;
+		translate: 0 -50%;
+		clip-path: polygon(100% 50%, 0 0, 0 100%);
+	}
+
+	.tooltip-bubble[data-arrow][data-resolved-side="right"]::after {
+		right: 100%;
+		top: var(--tooltip-arrow);
+		width: 0.25rem;
+		height: 0.5rem;
+		translate: 0 -50%;
+		clip-path: polygon(0 50%, 100% 0, 100% 100%);
+	}
+
+	.tooltip-bubble {
+		opacity: 0;
+		transition:
+			opacity 100ms ease-out,
+			overlay 100ms allow-discrete,
+			display 100ms allow-discrete;
+	}
+
+	.tooltip-bubble:popover-open {
+		opacity: 1;
+	}
+
+	@starting-style {
+		.tooltip-bubble:popover-open {
+			opacity: 0;
+		}
+	}
+
+	@media (prefers-reduced-motion: reduce) {
+		.tooltip-bubble {
+			transition: none;
+		}
+	}
+</style>