classroomio/prd/scroll-to-top

Scroll to Top

Status

• Draft (spec only; not implemented)

Purpose

Give learners (and anyone else reading a long page) a single, shared way to jump back to the top after they have scrolled away from it. The control is a floating button in the bottom-right corner. It is hidden until the page actually overflows and the user has scrolled a meaningful distance. Clicking it scrolls to the top. That is the entire interaction.

This is a shared UI convention, not a per-page invention. Any surface whose main column can grow taller than the viewport must use this component. Do not build a one-off back-to-top button.

Motivation

Course-taking pages are the longest surfaces in the product:

• Public course lessons and exercises (/course/[slug]/lesson/[itemSlug]) — rich-text bodies, video, questions, then a footer prev/next block.
• Authenticated LMS content list (/courses/[id]/lessons) — many sections and items; teachers and students both scroll the outline.
• Authenticated LMS lesson (/courses/[id]/lessons/[lessonId]) and exercise (/courses/[id]/exercises/[exerciseId]) — same long-read / long-form take experience inside the course shell.

Today there is no way back to the top except manually scrolling. On a phone, after reading a long lesson, that is a lot of work. The same gap will keep appearing on every new long page unless the pattern is a named component plus an AGENTS.md rule.

Confirmed decisions

1. One shared component in @cio/ui (packages/ui/src/custom/scroll-to-top). Dashboard, public course, and Storybook all consume it. Copy is passed in as a label prop (UI package stays English-free).
2. Hidden by default. The button is not in the visual tree at scrollTop === 0, even on a long page.
3. Appears only when both are true:
   • The scroll container overflows (content is taller than the viewport / container).
   • The user has scrolled past the show threshold.
4. Show threshold = one viewport height. scrollTop >= clientHeight of the scroll container. See Threshold recommendation.
5. Click does one thing: smooth-scroll that container to top: 0. No hash change, no extra menu, no “scroll to previous section”.
6. Fixed to the viewport, bottom-right. Not sticky inside the content column. Hosts that already have bottom chrome (public-course mobile nav, Ask AI bar) pass a clearance so the button sits above that chrome.
7. Mount in shells for v1, not on every leaf page. Visibility is still automatic: short pages inside those shells never show the button.
8. Not a global app layout mount. Auth screens, short admin lists, and dialogs must not pick it up by accident.

Threshold recommendation

Show after one viewport of scroll. Hide again below 0.8 viewports.

Option	Show when	Verdict
1 viewport (scrollTop >= clientHeight)	User has left the first screen	Choose this. Device-relative; late enough that a short bump does not flash the button; early enough on a 10-screen lesson.
Fixed 400px	Same distance on every device	Reject. ~½ screen on desktop, more than a screen on a short phone.
25–50% of document height	Proportion of content	Reject. A long lesson would hide the button until the user is already deep in.
Any overflow (show as soon as scrollTop > 0)	First pixel of scroll	Reject. Flickers on tiny overscroll / rubber-banding.

Hysteresis: show at 1.0 × viewport, hide at 0.8 × viewport, so sitting on the boundary does not strobe the button.

If the page cannot scroll a full viewport (content is only slightly taller than the window), the button never appears. That matches “only when there is real overflow.”

Constants (implementation must not scatter magic numbers):

export const SCROLL_TO_TOP_SHOW_RATIO = 1;
export const SCROLL_TO_TOP_HIDE_RATIO = 0.8;

Hosts may pass thresholdPx only for tests or genuine exceptions. Product default is the ratio above.

User stories

• As a learner on a long public lesson, after I have scrolled past the first screen, I want a button in the bottom-right so I can return to the title / video without dragging back.
• As a learner taking an LMS exercise with many questions or sections, I want the same control so I can jump back to the exercise heading.
• As a teacher or student on the course contents list with many sections, I want the same control after I have scrolled the outline.
• As a developer adding a new long-read page, I want one component and an AGENTS.md rule so I do not invent a fourth back-to-top button.

Feature detail

Component

Path: packages/ui/src/custom/scroll-to-top/

Export: ScrollToTop from @cio/ui/custom/scroll-to-top (and re-export from the @cio/ui barrel, same as IconButton).

Visual

• Icon-only circular (or rounded-full) control.
• Lucide ArrowUp (or ChevronUp) icon.
• Built on existing Button / IconButton: size="icon" (or icon-lg if tap target feels tight on mobile) and variant="secondary" — icon-only buttons must use secondary.
• Light shadow so it reads as floating above the page (ui:shadow-md or equivalent).
• Fade + short translate on enter/leave. If prefers-reduced-motion: reduce, skip the motion and scroll with behavior: 'instant'.
• Tooltip / aria-label from the label prop (required).

Default placement

viewport
┌─────────────────────────────┐
│                             │
│                             │
│                             │
│                             │
│                      ┌───┐  │  ← ~1.5rem from right and bottom
│                      │ ↑ │  │     (plus safe-area inset)
│                      └───┘  │
└─────────────────────────────┘

• position: fixed
• z-index: z-30 — above page content and the public-course sticky header (z-30 tie: later in DOM wins). Below dialogs/sheets (z-50+). Public-course mobile BottomNav is z-40; the button must clear it spatially, not compete in z-index (see Bottom chrome).
• Default inset: right: 1.5rem; bottom: max(1.5rem, env(safe-area-inset-bottom) + 1rem).

Behavior

1. Resolve the scroll container: window / document.documentElement by default, or an element passed as target.
2. On scroll (and on resize / content mutation): compute scrollTop, clientHeight, scrollHeight.
3. If scrollHeight <= clientHeight, stay hidden.
4. If scrollTop >= SHOW_RATIO * clientHeight, show. If scrollTop < HIDE_RATIO * clientHeight, hide.
5. On click: container.scrollTo({ top: 0, behavior }). Nothing else.
6. On SvelteKit navigation, re-evaluate (new page starts hidden). Do not leak a visible button across routes.

Listen with <svelte:window onscroll> / <svelte:window onresize> for the document case (do not wrap this in onMount + addEventListener). For an element target, attach onscroll to that node. Throttle visual updates with requestAnimationFrame if needed; do not put scroll-position writes inside $effect.

Props

Prop	Type	Default	Purpose
label	string	required	Accessible name and tooltip. Dashboard passes $t('common.scroll_to_top'). Public course / Storybook pass a literal or translated string from the host.
target	HTMLElement | null	null (window)	Nested overflow container, if the page does not scroll the document.
clearance	'default' | 'mobile-nav' | 'ask-ai'	'default'	Bottom offset preset for existing chrome.
class	string	—	Escape hatch for one-off offset. Prefer clearance.

Do not add an open / visible prop for product use. Visibility is derived from scroll. Storybook may use a forced-visible story via a dedicated forceVisible prop that is not used in app code.

Bottom chrome

The button must never sit under another fixed footer.

Surface	Existing chrome	clearance
Public course lesson/exercise, lg:hidden	PublicCourse.BottomNav (fixed inset-x-0 bottom-0 z-40; main already has pb-24)	mobile-nav → roughly bottom-24 on small screens, default on lg+
LMS lesson / exercise	ContentAskAiBar is centered at bottom-4 / full-width when expanded	ask-ai → lift on small screens so the FAB is not on top of the collapsed Ask AI chip; desktop can stay bottom-6 because Ask AI is centered and the FAB is right-aligned
Course contents list, most other pages	None at the bottom-right	default
Settings pages with Page.SettingsActions	Centered sticky card at z-50	default (right side is free). Do not cover the card.

Public-course shell should pass clearance="mobile-nav" itself so leaf lesson/exercise views stay unaware of the nav.

v1 mount points

Mount once per shell, not on every page file.

1. Public course shell

packages/ui/src/custom/public-course/shell.svelte

Covers:

• Public lesson view
• Public exercise view

(apps/dashboard/src/routes/(org-site)/course/[slug]/lesson/[itemSlug]/+page.svelte already renders inside this shell.)

Window is the scroller (min-h-dvh, no overflow trap on <main>). Default target is correct.

2. Authenticated course layout

apps/dashboard/src/routes/(app)/courses/[id]/+layout.svelte

Covers:

• Contents / lessons list — /courses/[id]/lessons (flat items or grouped sections; this is the “sections / contents” page)
• Lesson — /courses/[id]/lessons/[lessonId]
• Exercise — /courses/[id]/exercises/[exerciseId]
• Other course tabs that happen to be long (people, newsfeed, analytics) get the same automatic behavior; that is acceptable

The dashboard host supplies the translated label. The layout already knows when Ask AI is shown (showContentAskAiBar); pass clearance="ask-ai" on lesson/exercise routes and default otherwise.

Course pages scroll the document today (Sidebar.Provider is max-h-svh but the inset does not use overflow-y-auto). If that ever changes to an inner scroller, pass that element as target from the layout — do not fork the component.

Dashboard copy

Add to apps/dashboard/src/lib/utils/translations/en.json:

"common": {
  "scroll_to_top": "Back to top"
}

Run pnpm translate after adding the English key. Public course shell lives in @cio/ui and has no i18n: the org-site layout/page that constructs PublicCourseShell must pass the translated label (same pattern as exploreLabel / signInLabel).

Storybook

packages/storybook/src/molecules/scroll-to-top/scroll-to-top.stories.svelte

Stories:

1. In overflowing container — a tall target box; scrolling past one container-height reveals the button; click returns to top.
2. No overflow — short box; button never appears.
3. Forced visible — layout/docs only, so authors can see the default, mobile-nav, and ask-ai offsets without scrolling.

Agent / docs updates (this spec’s companion)

Implementation must keep these in sync:

• AGENTS.md — “Scroll to top” section (already added with this spec; tighten the import path if it moves).
• packages/ui/README.md — component section with props, clearance, and “use on any overflowing main column”.
• packages/ui/src/index.ts — barrel export.

Out of scope (v1)

• Scroll-spy, section dots, or “back to previous heading”.
• Remembering scroll position across lessons.
• Showing the button inside dialogs, sheets, drawers, or nested panes (transcript, comment trees, selects). Those keep their own scroll.
• A global mount on (app)/+layout.svelte or (org-site)/+layout.svelte.
• Changing Page.SettingsActions or Ask AI layout.
• Analytics on click (add later if we care about usage).

Later / same component, more hosts

Do not create a second control. When these pages are long, drop in ScrollToTop:

• Public course landing (/course/[slug])
• Org landing / catalog
• LMS community thread
• Course newsfeed (already under the course layout, so v1 may already cover it)
• Long settings forms (button at bottom-right; save bar stays centered)

Implementation checklist

• packages/ui/src/custom/scroll-to-top/ component + constants + ui: prefix
• Storybook stories (overflow, no overflow, clearance variants)
• packages/ui/README.md + barrel export
• Mount in PublicCourse.PublicCourseShell with clearance="mobile-nav" and a label prop on the shell
• Mount in courses/[id]/+layout.svelte with translated label and Ask AI clearance
• common.scroll_to_top + pnpm translate
• Confirm AGENTS.md still matches the real import path
• Manual pass: public lesson, public exercise, LMS contents list with many sections, LMS lesson, LMS exercise — button hidden at top, visible after ~1 screen, click returns to top, no overlap with BottomNav / Ask AI

Testing notes

• Public course: http://localhost:5173/course/<slug>/lesson/<itemSlug>?org=<siteName> (cloud VM: ?org=udemy-test).
• LMS: log in (admin@test.com / 123456), open a course → Lessons (contents) → a lesson → an exercise.
• Resize across the lg breakpoint on a public lesson: button must clear the mobile bottom nav and sit at the default offset on desktop.
• prefers-reduced-motion: no enter animation; scroll is instant.