fix(webapp): correct backward pagination slice in listRunIds

Backward pagination dropped the wrong end of the result set. listRunRows
fetches page.size + 1 rows to detect hasMore; that extra row is the one
farthest from the cursor in both directions (forward orders DESC,
backward orders ASC), so it is always the trailing element. Forward
correctly sliced rows[0..size); backward+hasMore sliced rows[1..size+1],
dropping the row closest to the cursor and keeping the has-more
sentinel. The result was a page that straddled two logical pages (one
run from the correct previous page plus one from the page before it) —
so paging "newer" across a boundary repeated and skipped runs.

Both directions now slice rows[0..size). Forward pagination, the
hasMore=false backward path, and all cursor values were already correct
and are unchanged.

Adds a regression test that walks forward across multiple pages, then
walks backward from the last page and asserts each backward page exactly
reproduces the corresponding forward page (and the full traversal covers
every run once).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: matt-aitken

.server-changes/runs-backward-pagination-slice.md

@@ -0,0 +1,14 @@
+---
+area: webapp
+type: fix
+---
+
+Fix an off-by-one in `ClickHouseRunsRepository.listRunIds` backward pagination.
+When paging backward with more rows before the page (`hasMore`), the displayed
+page was sliced as `rows.slice(1, size + 1)`, which dropped the row closest to
+the cursor and kept the extra "has-more" sentinel — returning a page that
+straddled two logical pages (one row from the correct previous page plus one
+from the page before it). The result set is always the first `page.size` rows
+(the sentinel is the trailing element in both directions), so the slice is now
+`rows.slice(0, size)` for forward and backward alike. Forward pagination and the
+cursor values were already correct and are unchanged.

apps/webapp/app/services/runsRepository/clickhouseRunsRepository.server.ts

@@ -123,11 +123,13 @@ export class ClickHouseRunsRepository implements IRunsRepository {
       }
     }
 
-    const runIds = (
-      direction === "backward" && hasMore
-        ? rows.slice(1, options.page.size + 1)
-        : rows.slice(0, options.page.size)
-    ).map((row) => row.runId);
+    // The page is always the first `page.size` rows of the result. listRunRows
+    // fetches one extra row only to detect `hasMore`; that extra row is the
+    // farthest from the cursor in BOTH directions (forward orders DESC, backward
+    // orders ASC), so it's always the trailing element to drop — never the
+    // leading one. (Slicing `[1, size+1]` for backward dropped the row closest
+    // to the cursor and kept the has-more sentinel, straddling two pages.)
+    const runIds = rows.slice(0, options.page.size).map((row) => row.runId);
 
     return { runIds, pagination: { nextCursor, previousCursor } };
   }

apps/webapp/test/runsRepositoryCursor.test.ts

@@ -296,4 +296,126 @@ describe("RunsRepository cursor pagination", () => {
       ]);
     }
   );
+
+  replicationContainerTest(
+    "backward pagination across multiple pages returns each page intact (no straddling)",
+    async ({ clickhouseContainer, redisOptions, postgresContainer, prisma }) => {
+      const { clickhouse } = await setupClickhouseReplication({
+        prisma,
+        databaseUrl: postgresContainer.getConnectionUri(),
+        clickhouseUrl: clickhouseContainer.getConnectionUrl(),
+        redisOptions,
+      });
+
+      const organization = await prisma.organization.create({
+        data: { title: "test", slug: "test" },
+      });
+      const project = await prisma.project.create({
+        data: {
+          name: "test",
+          slug: "test",
+          organizationId: organization.id,
+          externalRef: "test",
+        },
+      });
+      const runtimeEnvironment = await prisma.runtimeEnvironment.create({
+        data: {
+          slug: "test",
+          type: "DEVELOPMENT",
+          projectId: project.id,
+          organizationId: organization.id,
+          apiKey: "test",
+          pkApiKey: "test",
+          shortcode: "test",
+        },
+      });
+
+      // Five runs so a backward page has more rows before it (hasMore === true),
+      // which is the case the off-by-one in the backward slice corrupts.
+      const ids = [
+        "aaaaaaaaaaaaaaaaaaaaaaaa",
+        "bbbbbbbbbbbbbbbbbbbbbbbb",
+        "cccccccccccccccccccccccc",
+        "dddddddddddddddddddddddd",
+        "eeeeeeeeeeeeeeeeeeeeeeee",
+      ];
+      const base = new Date("2026-06-04T16:55:07.000Z").getTime();
+      for (let i = 0; i < ids.length; i++) {
+        await prisma.taskRun.create({
+          data: {
+            id: ids[i],
+            createdAt: new Date(base + (ids.length - 1 - i) * 1000),
+            friendlyId: `run_${ids[i]}`,
+            taskIdentifier: "my-task",
+            payload: JSON.stringify({ foo: "bar" }),
+            traceId: `trace_${i}`,
+            spanId: `span_${i}`,
+            queue: "test",
+            runtimeEnvironmentId: runtimeEnvironment.id,
+            projectId: project.id,
+            organizationId: organization.id,
+            environmentType: "DEVELOPMENT",
+            engine: "V2",
+          },
+        });
+      }
+
+      await setTimeout(1000);
+
+      const runsRepository = new RunsRepository({ prisma, clickhouse });
+      const baseOptions = {
+        projectId: project.id,
+        environmentId: runtimeEnvironment.id,
+        organizationId: organization.id,
+      };
+
+      const sortIds = (page: string[]) => page.slice().sort();
+
+      // Walk every forward page, capturing the run ids and the previousCursor.
+      const forwardPages: Array<{ ids: string[]; previousCursor: string | null }> = [];
+      let cursor: string | undefined = undefined;
+      for (let guard = 0; guard < 20; guard++) {
+        const page = await runsRepository.listRuns({
+          ...baseOptions,
+          page: { size: 2, cursor, direction: cursor ? "forward" : undefined },
+        });
+        forwardPages.push({
+          ids: page.runs.map((r) => r.id),
+          previousCursor: page.pagination.previousCursor,
+        });
+        if (!page.pagination.nextCursor) break;
+        cursor = page.pagination.nextCursor;
+      }
+
+      // Forward pagination itself must cover every run exactly once, in 3 pages.
+      expect(forwardPages.flatMap((p) => p.ids).sort()).toEqual(ids.slice().sort());
+      expect(forwardPages).toHaveLength(3);
+
+      // Walk backward from the last page. Each backward page must equal the
+      // corresponding forward page exactly — no row from an adjacent page
+      // bleeding in (the straddle bug returned e.g. {b,c} instead of {c,d}).
+      const backwardPages: string[][] = [];
+      let backCursor: string | null = forwardPages[forwardPages.length - 1].previousCursor;
+      for (let guard = 0; guard < 20 && backCursor; guard++) {
+        const page = await runsRepository.listRuns({
+          ...baseOptions,
+          page: { size: 2, cursor: backCursor, direction: "backward" },
+        });
+        backwardPages.push(page.runs.map((r) => r.id));
+        backCursor = page.pagination.previousCursor;
+      }
+
+      const expectedBackward = forwardPages
+        .slice(0, -1)
+        .reverse()
+        .map((p) => sortIds(p.ids));
+      expect(backwardPages.map(sortIds)).toEqual(expectedBackward);
+
+      // And the full backward traversal (last page + everything walked back to
+      // the start) covers every run exactly once.
+      const seen = [...forwardPages[forwardPages.length - 1].ids, ...backwardPages.flat()];
+      expect(seen.slice().sort()).toEqual(ids.slice().sort());
+      expect(new Set(seen).size).toBe(ids.length);
+    }
+  );
 });