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Don’t sync co-reading by copying one window’s scrollY into another. Treat the reader’s position as transient presence. Describe it in a form that survives different viewports and layouts. Keep it separate from the document’s durable content. Give each reader a clear way to stop following someone else. The Yjs and MDN documentation supply the building blocks for this. Neither describes a finished co-reading feature, so the anchor and loop-prevention advice below is engineering inference, not documented behavior.
Three kinds of state, three lifetimes
A co-reading session mixes data with very different lifespans. Mixing them in one store is the most common design mistake.
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| State | Examples | Lifetime | Suitable home |
|---|---|---|---|
| Shared content | Text, annotations, edits | Persistent | Collaborative document model |
| Live presence | Who is here, active section, current position, follow status | Ephemeral | Presence channel |
| Bookmark | “Resume where I left off” | Persistent, deliberate | Separate product feature |
Yjs follows this split. Its documents merge updates across clients: the documentation describes updates as commutative, associative, and idempotent, with state vectors for sending only missing differences. That solves convergence of shared content. It does not define how a reading position should be represented.
For live position, Yjs offers the optional Awareness mechanism. Its tutorial states: “Awareness information isn’t stored in the Yjs document, as it doesn’t need to be persisted across sessions.” (Awareness & Presence). The Awareness API page documents that peers are treated as offline after 30 seconds without updates. That is Yjs’s own behavior, not a general standard for real-time collaboration. A saved bookmark is a different thing and should be stored on purpose.
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Choosing what “same place” means
Decide this before writing any code. It determines the position format.
Pixel offset or normalized progress
Sending a scroll offset, or a 0–1 progress value, is simple and works when both readers see identical content at identical layout. Offsets diverge when font size, viewport width, responsive breakpoints or dynamically loaded content differ, because the same pixel value then lands in different text.
Content anchor plus local offset
Sending a stable section or paragraph ID with an offset inside it can mean more across layouts. The cost is that you need stable identifiers and code to resolve and restore the anchor. Neither Yjs nor the browser APIs provide this abstraction. It is a recommendation inferred from the limits of pixel offsets, so test it against reflow and content edits before relying on it.
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Section-level presence
If readers only need to know roughly where others are, report the active section and skip continuous alignment. This is the cheapest option and often enough.
Observing the reader without causing jank
The scroll event
MDN states that the document scroll event fires when the document view has been scrolled, and that it can fire at a high rate. Keep the handler cheap. If you need to throttle, MDN advises using a measured timeout instead of assuming requestAnimationFrame() reduces event frequency. Reading a position and queuing a small message is fine. Layout-heavy DOM work per event is not.
Intersection Observer for sections
Intersection Observer reports asynchronously when elements cross thresholds you configure. It is well suited to “which section is this reader in.” Two limits matter. Entries describe a point-in-time transition, and the API does not tell you the exact number of overlapping pixels. It is therefore not a full replacement for pixel-level tracking. Choose thresholds that match your idea of “reading this section” rather than “one pixel is visible.”
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Applying remote positions safely
The sources show that awareness changes propagate to other clients and that scroll handlers fire often. They do not give a loop-prevention recipe, so treat the following as design guidance to verify in your own build.
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- Flag programmatic scrolls. When you scroll a follower to a remote position, mark that movement so the scroll handler does not broadcast it as the follower’s own reading.
- Don’t forward every raw event. Send updates at a measured interval or when the active anchor changes.
- Separate follow from browse. Following another reader is a mode. A manual scroll by the follower should exit it or prompt them, and a visible control should let them leave.
Follow models
These are product decisions; none is specified by the tools.
- Leader mode: everyone tracks one reader. It suits presentations and is the most intrusive.
- Optional follow: readers choose whom to follow and can stop. This is the safest default for general reading.
- Presence only: show where others are, such as an avatar beside a section, and never move anyone’s view.
Transport: same-browser versus across devices
BroadcastChannel passes messages among eligible documents and workers in the same origin and storage partition. It helps with several tabs on one device, and does not connect readers on separate devices.
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For separate readers you need a network provider. Yjs is network-agnostic. Its collaborative editor guide describes WebRTC as handy for demos without an application server, while noting that real applications often want server synchronization. The Yjs introduction lists hosted options such as Liveblocks, Y-Sweet and Tiptap. Treat these as candidates to evaluate. The documentation reviewed here does not compare their features, prices, service levels or security terms.
Evaluation checklist
- State semantics: is each field edit, presence or bookmark?
- Position fidelity: pixels, progress, section or anchor?
- Network shape: peer-to-peer or server-mediated, and how do you recover from disconnects and persist data?
- Reader control: always-follow, opt-in, or presence-only, with an obvious exit?
- Workload: concurrent readers, update rate and payload size. The cited sources give no capacity figures for this use case, so measure.
- Privacy and retention: what identity and position are shared, with whom, and whether history is stored. The documentation does not settle policy or legal requirements.
What remains unproven
No standard co-reading position format exists in the sources. They also give no measured sync latency, no behavior under changing layouts, and no provider performance comparison. If you claim any of those for your product, validate them with your own tests.
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