fix(relations): improve thread pagination and include root event

Replace unreliable PduCount pagination tokens with ShortEventId throughout
the relations and messages endpoints. ShortEventId provides stable, unique
identifiers that persist across server restarts and database operations.

Key improvements:
- Add token parsing helpers that try ShortEventId first, fall back to
  PduCount for backwards compatibility
- Include thread root event when paginating backwards to thread start
- Fix off-by-one error in get_relations that was returning the starting
  event in results
- Only return next_batch/prev_batch tokens when more events are available,
  preventing clients from making unnecessary requests at thread boundaries
- Ensure consistent token format between /relations, /messages, and /sync
  endpoints for interoperability

This fixes duplicate events when scrolling at thread boundaries and ensures
the thread root message is visible when viewing a thread, matching expected
client behaviour.

src/api/client/message.rs

@@ -1,14 +1,14 @@
 use axum::extract::State;
 use conduwuit::{
-	Err, Result, at,
+	Err, Result, at, err,
 	matrix::{
 		event::{Event, Matches},
-		pdu::PduCount,
+		pdu::{PduCount, ShortEventId},
 	},
 	ref_at,
 	utils::{
 		IterStream, ReadyExt,
-		result::{FlatOk, LogErr},
+		result::LogErr,
 		stream::{BroadbandExt, TryIgnore, WidebandExt},
 	},
 };
@@ -61,6 +61,39 @@ const IGNORED_MESSAGE_TYPES: &[TimelineEventType] = &[
 const LIMIT_MAX: usize = 100;
 const LIMIT_DEFAULT: usize = 10;
 
+/// Parse a pagination token, trying ShortEventId first, then falling back to
+/// PduCount
+async fn parse_pagination_token(
+	_services: &Services,
+	_room_id: &RoomId,
+	token: Option<&str>,
+	default: PduCount,
+) -> Result<PduCount> {
+	let Some(token) = token else {
+		return Ok(default);
+	};
+
+	// Try parsing as ShortEventId first
+	if let Ok(shorteventid) = token.parse::<ShortEventId>() {
+		// ShortEventId maps directly to a PduCount in our database
+		Ok(PduCount::Normal(shorteventid))
+	} else if let Ok(count) = token.parse::<u64>() {
+		// Fallback to PduCount for backwards compatibility
+		Ok(PduCount::Normal(count))
+	} else if let Ok(count) = token.parse::<i64>() {
+		// Also handle negative counts for backfilled events
+		Ok(PduCount::from_signed(count))
+	} else {
+		Err(err!(Request(InvalidParam("Invalid pagination token"))))
+	}
+}
+
+/// Convert a PduCount to a token string (using the underlying ShortEventId)
+fn count_to_token(count: PduCount) -> String {
+	// The PduCount's unsigned value IS the ShortEventId
+	count.into_unsigned().to_string()
+}
+
 /// # `GET /_matrix/client/r0/rooms/{roomId}/messages`
 ///
 /// Allows paginating through room history.
@@ -81,17 +114,18 @@ pub(crate) async fn get_message_events_route(
 		return Err!(Request(Forbidden("Room does not exist to this server")));
 	}
 
-	let from: PduCount = body
-		.from
-		.as_deref()
-		.map(str::parse)
-		.transpose()?
-		.unwrap_or_else(|| match body.dir {
+	let from: PduCount =
+		parse_pagination_token(&services, room_id, body.from.as_deref(), match body.dir {
 			| Direction::Forward => PduCount::min(),
 			| Direction::Backward => PduCount::max(),
-		});
+		})
+		.await?;
 
-	let to: Option<PduCount> = body.to.as_deref().map(str::parse).flat_ok();
+	let to: Option<PduCount> = if let Some(to_str) = body.to.as_deref() {
+		Some(parse_pagination_token(&services, room_id, Some(to_str), PduCount::min()).await?)
+	} else {
+		None
+	};
 
 	let limit: usize = body
 		.limit
@@ -180,8 +214,8 @@ pub(crate) async fn get_message_events_route(
 		.collect();
 
 	Ok(get_message_events::v3::Response {
-		start: from.to_string(),
-		end: next_token.as_ref().map(ToString::to_string),
+		start: count_to_token(from),
+		end: next_token.map(count_to_token),
 		chunk,
 		state,
 	})

src/api/client/relations.rs

@@ -1,7 +1,11 @@
 use axum::extract::State;
 use conduwuit::{
-	Result, at,
-	matrix::{Event, event::RelationTypeEqual, pdu::PduCount},
+	Result, at, err,
+	matrix::{
+		Event,
+		event::RelationTypeEqual,
+		pdu::{PduCount, ShortEventId},
+	},
 	utils::{IterStream, ReadyExt, result::FlatOk, stream::WidebandExt},
 };
 use conduwuit_service::Services;
@@ -20,6 +24,40 @@ use ruma::{
 
 use crate::Ruma;
 
+/// Parse a pagination token, trying ShortEventId first, then falling back to
+/// PduCount
+async fn parse_pagination_token(
+	_services: &Services,
+	_room_id: &RoomId,
+	token: Option<&str>,
+	default: PduCount,
+) -> Result<PduCount> {
+	let Some(token) = token else {
+		return Ok(default);
+	};
+
+	// Try parsing as ShortEventId first
+	if let Ok(shorteventid) = token.parse::<ShortEventId>() {
+		// ShortEventId maps directly to a PduCount in our database
+		// The shorteventid IS the count value, just need to wrap it
+		Ok(PduCount::Normal(shorteventid))
+	} else if let Ok(count) = token.parse::<u64>() {
+		// Fallback to PduCount for backwards compatibility
+		Ok(PduCount::Normal(count))
+	} else if let Ok(count) = token.parse::<i64>() {
+		// Also handle negative counts for backfilled events
+		Ok(PduCount::from_signed(count))
+	} else {
+		Err(err!(Request(InvalidParam("Invalid pagination token"))))
+	}
+}
+
+/// Convert a PduCount to a token string (using the underlying ShortEventId)
+fn count_to_token(count: PduCount) -> String {
+	// The PduCount's unsigned value IS the ShortEventId
+	count.into_unsigned().to_string()
+}
+
 /// # `GET /_matrix/client/r0/rooms/{roomId}/relations/{eventId}/{relType}/{eventType}`
 pub(crate) async fn get_relating_events_with_rel_type_and_event_type_route(
 	State(services): State<crate::State>,
@@ -109,15 +147,17 @@ async fn paginate_relations_with_filter(
 	recurse: bool,
 	dir: Direction,
 ) -> Result<get_relating_events::v1::Response> {
-	let start: PduCount = from
-		.map(str::parse)
-		.transpose()?
-		.unwrap_or_else(|| match dir {
+	let start: PduCount = parse_pagination_token(services, room_id, from, match dir {
 		| Direction::Forward => PduCount::min(),
 		| Direction::Backward => PduCount::max(),
-		});
+	})
+	.await?;
 
-	let to: Option<PduCount> = to.map(str::parse).flat_ok();
+	let to: Option<PduCount> = if let Some(to_str) = to {
+		Some(parse_pagination_token(services, room_id, Some(to_str), PduCount::min()).await?)
+	} else {
+		None
+	};
 
 	// Use limit or else 30, with maximum 100
 	let limit: usize = limit
@@ -129,6 +169,11 @@ async fn paginate_relations_with_filter(
 	// Spec (v1.10) recommends depth of at least 3
 	let depth: u8 = if recurse { 3 } else { 1 };
 
+	// Check if this is a thread request
+	let is_thread = filter_rel_type
+		.as_ref()
+		.is_some_and(|rel| *rel == RelationType::Thread);
+
 	let events: Vec<_> = services
 		.rooms
 		.pdu_metadata
@@ -152,23 +197,65 @@ async fn paginate_relations_with_filter(
 		.collect()
 		.await;
 
-	let next_batch = match dir {
+	// For threads, check if we should include the root event
+	let mut root_event = None;
+	if is_thread && dir == Direction::Backward {
+		// Check if we've reached the beginning of the thread
+		// (fewer events than requested means we've exhausted the thread)
+		if events.len() < limit {
+			// Try to get the thread root event
+			if let Ok(root_pdu) = services.rooms.timeline.get_pdu(target).await {
+				// Check visibility
+				if services
+					.rooms
+					.state_accessor
+					.user_can_see_event(sender_user, room_id, target)
+					.await
+				{
+					// Store the root event to add to the response
+					root_event = Some(root_pdu);
+				}
+			}
+		}
+	}
+
+	// Determine if there are more events to fetch
+	let has_more = if root_event.is_some() {
+		false // We've included the root, no more events
+	} else {
+		// Check if we got a full page of results (might be more)
+		events.len() >= limit
+	};
+
+	let next_batch = if has_more {
+		match dir {
 			| Direction::Forward => events.last(),
 			| Direction::Backward => events.first(),
 		}
-	.map(at!(0))
-	.as_ref()
-	.map(ToString::to_string);
+		.map(|(count, _)| count_to_token(*count))
+	} else {
+		None
+	};
+
+	// Build the response chunk with thread root if needed
+	let chunk: Vec<_> = if let Some(root) = root_event {
+		// Add root event at the beginning for backward pagination
+		std::iter::once(root.into_format())
+			.chain(events.into_iter().map(at!(1)).map(Event::into_format))
+			.collect()
+	} else {
+		events
+			.into_iter()
+			.map(at!(1))
+			.map(Event::into_format)
+			.collect()
+	};
 
 	Ok(get_relating_events::v1::Response {
 		next_batch,
 		prev_batch: from.map(Into::into),
 		recursion_depth: recurse.then_some(depth.into()),
-		chunk: events
-			.into_iter()
-			.map(at!(1))
-			.map(Event::into_format)
-			.collect(),
+		chunk,
 	})
 }
 

src/service/rooms/pdu_metadata/data.rs

@@ -61,9 +61,10 @@ impl Data {
 		from: PduCount,
 		dir: Direction,
 	) -> impl Stream<Item = (PduCount, impl Event)> + Send + '_ {
+		let from_unsigned = from.into_unsigned();
 		let mut current = ArrayVec::<u8, 16>::new();
 		current.extend(target.to_be_bytes());
-		current.extend(from.saturating_inc(dir).into_unsigned().to_be_bytes());
+		current.extend(from_unsigned.to_be_bytes());
 		let current = current.as_slice();
 		match dir {
 			| Direction::Forward => self.tofrom_relation.raw_keys_from(current).boxed(),
@@ -73,6 +74,17 @@ impl Data {
 		.ready_take_while(move |key| key.starts_with(&target.to_be_bytes()))
 		.map(|to_from| u64_from_u8(&to_from[8..16]))
 		.map(PduCount::from_unsigned)
+		.ready_filter(move |count| {
+			if from == PduCount::min() || from == PduCount::max() {
+				true
+			} else {
+				let count_unsigned = count.into_unsigned();
+				match dir {
+					| Direction::Forward => count_unsigned > from_unsigned,
+					| Direction::Backward => count_unsigned < from_unsigned,
+				}
+			}
+		})
 		.wide_filter_map(move |shorteventid| async move {
 			let pdu_id: RawPduId = PduId { shortroomid, shorteventid }.into();