Each target's row shows its result as soon as its check ends (closes #91)

tick drew no row until the round's slowest check ended, up to 24
seconds at a 30-second interval since checks time out at 80% of it.
Each check now pushes its sample and redraws its row as it ends;
sorting, the summary, the health box and offline detection still run
once, after the last check. A check that ends while paused or after its
round is given up draws nothing, and the first round is still discarded
as a whole. The row is looked up when the check ends, as a pin click
can re-sort the rows mid-round.

The new unit test runs tick on the mocked clock against a stand-in
page.

Model: opus-5-5

Dockerfile

@@ -20,7 +20,10 @@ RUN make lint
 # golang:1.25-alpine (2026-02-27)
 FROM golang:1.25-alpine@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
 
-RUN apk add --no-cache git make
+# gcc and musl-dev are for make test: its race detector needs cgo, which
+# Go turns on by itself once a C compiler is present. make build still
+# sets CGO_ENABLED=0, so the binary stays static.
+RUN apk add --no-cache gcc git make musl-dev
 
 WORKDIR /src
 

README.md

@@ -39,14 +39,16 @@ halves, so the root `make check` fails if either one is broken. We provide:
 - `script/bootstrap` — install all dependencies (the pinned node via nvm unless
   one new enough for the frontend's dependencies is installed, yarn via
   corepack, `yarn install --frozen-lockfile`, the pinned Go unless one at least
-  as new as `backend/go.mod` asks for is installed, and the Go modules), linking
-  what it installs itself into `~/.local/bin`, which has to be on `PATH`. It
-  installs no Go linter and not Docker: `make lint` runs the linter in Docker
+  as new as `backend/go.mod` asks for is installed, the Go modules, and gcc with
+  the C library headers unless gcc is installed, for the race detector in
+  `make test`), linking what it installs itself into `~/.local/bin`, which has
+  to be on `PATH`. It installs no Go linter and not Docker: `make lint` runs the
+  linter in Docker
 - `script/setup` — make a fresh clone ready for development: bootstrap plus the
   git pre-commit hook
 - `script/projectname` — print the project name (used for the Docker image tag)
-- `script/test` — run `script/frontend-test`, then the backend's Go tests, both
-  within one 30-second timeout
+- `script/test` — run `script/frontend-test`, then the backend's Go tests, each
+  under its own 30-second timeout
 - `script/lint` — run `script/frontend-lint`, then golangci-lint in Docker, by
   building the lint stage of `Dockerfile` without the cache
 - `script/fmt` — format all files (writes): prettier, then gofmt over `backend/`
@@ -102,9 +104,10 @@ code lives in `src/main.js` with a class-based architecture:
   color-coded line segments, error regions, and DPR-aware scaling
 - **UI functions**: `buildUI()` constructs the DOM, `updateHostRow()` /
   `updateSummary()` / `updateHealthBox()` handle incremental updates
-- **`tick()`**: Main loop — measures all hosts in parallel via `Promise.all`,
-  pushes samples, redraws UI. When paused, pushes blank markers (no probes, no
-  false outage)
+- **`tick()`**: Main loop — measures all hosts in parallel, pushing each host's
+  sample and redrawing its row as soon as its check ends, then sorts and redraws
+  the summary and health box once the last check ends. When paused, pushes blank
+  markers (no probes, no false outage)
 - **`Reporter`**: Posts collected samples to the backend
 
 ### Reporting
@@ -142,8 +145,8 @@ have all finished before the next round is due. When no WAN host answers, a
 recovery probe checks 4 random WAN hosts every half second, giving up the checks
 it started half a second before. As soon as one answers, a new round starts at
 once, as it does after an interval change. A round started early gives up the
-last round's checks if they are still waiting, and that round records nothing,
-so rounds never overlap. IPv4 only.
+last round's checks if they are still waiting, and that round records nothing
+more, so rounds never overlap. IPv4 only.
 
 ### Color coding
 

TODO.md

@@ -23,6 +23,23 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-10-03: each target's row shows its result as soon as its check ends
+  (issue #91), where every row waited for the round's slowest check, up to 24
+  seconds at a 30-second interval. Sorting, the summary, the health box and
+  offline detection still run once, when the round's last check ends. A check
+  that ends after the user pauses or after its round is given up shows nothing,
+  and the first round is still discarded as a whole
+- 2026-10-03: the Go tests run with the race detector and coverage (issue #88):
+  `backend/script/test` runs `go test -timeout 30s -race -cover ./...` and, if
+  that fails, runs it again with `-v` and fails. Go's `-timeout` bounds the
+  tests, not their compile; the root `script/test` no longer puts one 30-second
+  timeout around both halves, which a cold Go build cache could use up on
+  compiling alone. The race detector needs a C compiler: the builder stage of
+  `Dockerfile` has gcc and musl-dev, and `script/bootstrap` installs gcc, with
+  the C library headers on apt and apk, when gcc is missing; the binary is still
+  built with `CGO_ENABLED=0`. New tests cover the health check's answer, a valid
+  report's answer, a report file's exact contents, and the flush when the buffer
+  reaches 10 MiB; the handlers' `TestImport` stub is gone
 - 2026-10-03: each target check times out after 80% of the refresh interval
   (issue #78), 24 seconds at 30 seconds, where it was capped at 3 seconds. A
   round started early, after an interval change or when the recovery probe finds

backend/README.md

@@ -35,7 +35,10 @@ pattern as the repo root: the targets in `backend/Makefile` are thin shims over
   stamped in. The version is `VERSION` from the environment;
   when that is unset or empty, it falls back to `git describe` inside a git
   checkout, then to `dev`
-- `script/test` — run the Go tests under a 30-second timeout
+- `script/test` — run the Go tests with the race detector and coverage. Go's
+  `-timeout 30s` bounds the tests, not their compile. If they fail, they run
+  again with `-v` for the details, and the script fails. The race detector needs
+  a C compiler
 - `script/lint` — check `.golangci.yml` against its pinned sha256, then run
   golangci-lint. It runs inside the golangci-lint image of the lint stage of
   the root `Dockerfile`; from a checkout, run `make lint` at the repo root,

backend/internal/handlers/handlers_test.go

@@ -1,13 +0,0 @@
-package handlers_test
-
-import (
-	"testing"
-
-	_ "sneak.berlin/go/netwatch/internal/handlers"
-)
-
-func TestImport(t *testing.T) {
-	t.Parallel()
-	// Compilation check — verifies the package parses
-	// and all imports resolve.
-}

backend/internal/handlers/healthcheck_test.go

@@ -0,0 +1,120 @@
+package handlers_test
+
+import (
+	"encoding/json"
+	"maps"
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/netwatch/internal/globals"
+	"sneak.berlin/go/netwatch/internal/handlers"
+	"sneak.berlin/go/netwatch/internal/healthcheck"
+	"sneak.berlin/go/netwatch/internal/logger"
+
+	"go.uber.org/fx/fxtest"
+)
+
+// newStartedHandlers builds Handlers with a real health check for the
+// server named in g, and starts them, which records the time the
+// uptime counts from.
+func newStartedHandlers(t *testing.T, g *globals.Globals) *handlers.Handlers {
+	t.Helper()
+
+	lc := fxtest.NewLifecycle(t)
+
+	log, err := logger.New(lc, logger.Params{Globals: g})
+	if err != nil {
+		t.Fatalf("logger: %v", err)
+	}
+
+	hc, err := healthcheck.New(lc,
+		healthcheck.Params{Globals: g, Logger: log})
+	if err != nil {
+		t.Fatalf("health check: %v", err)
+	}
+
+	h, err := handlers.New(lc,
+		handlers.Params{Globals: g, Healthcheck: hc, Logger: log})
+	if err != nil {
+		t.Fatalf("handlers: %v", err)
+	}
+
+	lc.RequireStart()
+	t.Cleanup(lc.RequireStop)
+
+	return h
+}
+
+// TestHandleHealthCheck checks the health check's answer: 200, a JSON
+// content type, and a JSON object with exactly the fields of
+// healthcheck.Response, carrying this server's name and version and
+// an uptime counted from its start.
+func TestHandleHealthCheck(t *testing.T) {
+	t.Parallel()
+
+	g := &globals.Globals{Appname: "netwatch-server", Version: "v1.2.3"}
+	h := newStartedHandlers(t, g)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodGet, "/.well-known/healthcheck", http.NoBody)
+
+	h.HandleHealthCheck().ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	contentType := rec.Header().Get("Content-Type")
+	if contentType != "application/json; charset=utf-8" {
+		t.Errorf("Content-Type = %q, want %q",
+			contentType, "application/json; charset=utf-8")
+	}
+
+	var body map[string]any
+
+	err := json.Unmarshal(rec.Body.Bytes(), &body)
+	if err != nil {
+		t.Fatalf("body not a JSON object: %v (%q)", err, rec.Body.String())
+	}
+
+	fields := []string{
+		"appname", "now", "status", "uptimeHuman", "uptimeSeconds", "version",
+	}
+	if got := slices.Sorted(maps.Keys(body)); !slices.Equal(got, fields) {
+		t.Fatalf("fields = %v, want %v", got, fields)
+	}
+
+	for field, want := range map[string]string{
+		"appname": g.Appname, "status": "ok", "version": g.Version,
+	} {
+		if body[field] != want {
+			t.Errorf("%s = %v, want %q", field, body[field], want)
+		}
+	}
+
+	now, _ := body["now"].(string)
+
+	at, err := time.Parse(time.RFC3339Nano, now)
+	if err != nil || time.Since(at).Abs() > time.Minute {
+		t.Errorf("now = %q, want the current time in RFC 3339 (%v)", now, err)
+	}
+
+	// Started just now, so the uptime is well under a minute.
+	human, _ := body["uptimeHuman"].(string)
+
+	uptime, err := time.ParseDuration(human)
+	if err != nil || uptime > time.Minute {
+		t.Errorf("uptimeHuman = %q, want a duration under a minute (%v)",
+			human, err)
+	}
+
+	seconds, ok := body["uptimeSeconds"].(float64)
+	if !ok || seconds < 0 || seconds > time.Minute.Seconds() {
+		t.Errorf("uptimeSeconds = %v, want a number of seconds under a minute",
+			body["uptimeSeconds"])
+	}
+}

backend/internal/handlers/report_test.go

@@ -46,6 +46,63 @@ func decodeStatus(t *testing.T, body []byte) string {
 	return resp.Status
 }
 
+// TestHandleReportAcceptsValidReports checks the answer to a valid
+// report, one with no hosts and one shaped as the frontend sends them:
+// 200 and {"status":"ok"} as JSON.
+func TestHandleReportAcceptsValidReports(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		body string
+	}{
+		{
+			name: "no hosts",
+			body: `{"clientId":"c1","geo":null,"hosts":[],` +
+				`"timestamp":"2026-10-03T12:00:00.000Z"}`,
+		},
+		{
+			name: "a host with a latency and an error sample",
+			body: `{"clientId":"c1","geo":null,"hosts":[{` +
+				`"name":"Example","url":"https://example.com/",` +
+				`"status":"error","history":[` +
+				`{"t":1790000000000,"latency":42,"error":null},` +
+				`{"t":1790000003000,"latency":null,"error":"timeout"}]}],` +
+				`"timestamp":"2026-10-03T12:00:00.000Z"}`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			h := newTestHandlers(stubAppender{}, io.Discard)
+
+			rec := httptest.NewRecorder()
+			req := httptest.NewRequestWithContext(t.Context(),
+				http.MethodPost, "/api/v1/reports",
+				strings.NewReader(tt.body),
+			)
+
+			h.HandleReport().ServeHTTP(rec, req)
+
+			if rec.Code != http.StatusOK {
+				t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+			}
+
+			contentType := rec.Header().Get("Content-Type")
+			if contentType != "application/json; charset=utf-8" {
+				t.Errorf("Content-Type = %q, want %q",
+					contentType, "application/json; charset=utf-8")
+			}
+
+			if got := rec.Body.String(); got != "{\"status\":\"ok\"}\n" {
+				t.Errorf("body = %q, want %q", got, "{\"status\":\"ok\"}\n")
+			}
+		})
+	}
+}
+
 func TestHandleReportStorageFailureIsNon2xx(t *testing.T) {
 	t.Parallel()
 

backend/internal/reportbuf/export_test.go

@@ -2,6 +2,10 @@ package reportbuf
 
 import "time"
 
+// FlushSizeThreshold exposes the buffer size at which Append starts
+// writing a report file to the external tests.
+const FlushSizeThreshold = flushSizeThreshold
+
 // Flush writes the buffered reports to a file now, as the periodic
 // flush does, so tests need not wait a minute for it.
 func (b *Buffer) Flush() error {

backend/internal/reportbuf/reportbuf_test.go

@@ -334,7 +334,11 @@ func TestTwoFlushesInOneMillisecond(t *testing.T) {
 		}
 	}
 
-	files := readReportFiles(t, dir)
+	files, err := readReportFiles(dir)
+	if err != nil {
+		t.Fatalf("read report files: %v", err)
+	}
+
 	if len(files) != flushes {
 		t.Fatalf("%d report files after %d flushes", len(files), flushes)
 	}
@@ -347,6 +351,88 @@ func TestTwoFlushesInOneMillisecond(t *testing.T) {
 	}
 }
 
+// TestReportFileHoldsTheLinesAppended flushes three reports and reads
+// their file back: it must decompress to exactly their JSON lines, in
+// the order they were appended.
+func TestReportFileHoldsTheLinesAppended(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("DATA_DIR", dir)
+
+	buf := startBuffer(t)
+
+	for _, id := range []int{1, 2, 3} {
+		err := buf.Append(map[string]int{"id": id})
+		if err != nil {
+			t.Fatalf("append report %d: %v", id, err)
+		}
+	}
+
+	err := buf.Flush()
+	if err != nil {
+		t.Fatalf("flush: %v", err)
+	}
+
+	files, err := readReportFiles(dir)
+	if err != nil {
+		t.Fatalf("read report files: %v", err)
+	}
+
+	want := `{"id":1}` + "\n" + `{"id":2}` + "\n" + `{"id":3}` + "\n"
+	if len(files) != 1 || files[0] != want {
+		t.Fatalf("report files = %q, want one holding %q", files, want)
+	}
+}
+
+// TestFlushAtSizeThreshold appends reports until the buffer holds
+// FlushSizeThreshold bytes. The append that gets it there must write
+// them all to one report file, with no call to Flush and the periodic
+// flush a minute away, and no earlier append may write one.
+func TestFlushAtSizeThreshold(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("DATA_DIR", dir)
+
+	buf := startBuffer(t)
+
+	// Large reports, so the threshold takes a few hundred appends.
+	pad := strings.Repeat("a", 64<<10)
+
+	var appended strings.Builder
+
+	for id := 0; appended.Len() < reportbuf.FlushSizeThreshold; id++ {
+		report := map[string]any{"id": id, "pad": pad}
+
+		err := buf.Append(report)
+		if err != nil {
+			t.Fatalf("append report %d: %v", id, err)
+		}
+
+		line, err := json.Marshal(report)
+		if err != nil {
+			t.Fatalf("marshal report %d: %v", id, err)
+		}
+
+		appended.Write(line)
+		appended.WriteByte('\n')
+	}
+
+	// Append writes the file in the background, so wait for it.
+	deadline := time.Now().Add(10 * time.Second)
+
+	for {
+		files, err := readReportFiles(dir)
+		if err == nil && len(files) == 1 && files[0] == appended.String() {
+			return
+		}
+
+		if time.Now().After(deadline) {
+			t.Fatalf("%d report files (error: %v), want one holding the "+
+				"%d bytes appended", len(files), err, appended.Len())
+		}
+
+		time.Sleep(10 * time.Millisecond)
+	}
+}
+
 // reportFilesBytes returns the total size of the report files in dir.
 func reportFilesBytes(t *testing.T, dir string) int64 {
 	t.Helper()
@@ -371,20 +457,19 @@ func reportFilesBytes(t *testing.T, dir string) int64 {
 }
 
 // readReportFiles returns the decompressed contents of each report
-// file in dir.
-func readReportFiles(t *testing.T, dir string) []string {
-	t.Helper()
-
+// file in dir. A file still being written does not decompress, so it
+// gives an error.
+func readReportFiles(dir string) ([]string, error) {
 	files := os.DirFS(dir)
 
 	names, err := fs.Glob(files, "reports-*.jsonl.zst")
 	if err != nil {
-		t.Fatalf("list report files: %v", err)
+		return nil, fmt.Errorf("list report files: %w", err)
 	}
 
 	dec, err := zstd.NewReader(nil)
 	if err != nil {
-		t.Fatalf("create zstd decoder: %v", err)
+		return nil, fmt.Errorf("create zstd decoder: %w", err)
 	}
 	defer dec.Close()
 
@@ -393,18 +478,18 @@ func readReportFiles(t *testing.T, dir string) []string {
 	for _, name := range names {
 		compressed, readErr := fs.ReadFile(files, name)
 		if readErr != nil {
-			t.Fatalf("read %s: %v", name, readErr)
+			return nil, fmt.Errorf("read %s: %w", name, readErr)
 		}
 
 		data, decErr := dec.DecodeAll(compressed, nil)
 		if decErr != nil {
-			t.Fatalf("decompress %s: %v", name, decErr)
+			return nil, fmt.Errorf("decompress %s: %w", name, decErr)
 		}
 
 		contents = append(contents, string(data))
 	}
 
-	return contents
+	return contents, nil
 }
 
 func writeBytes(t *testing.T, path string, n int) {

backend/script/test

@@ -1,12 +1,20 @@
 #!/bin/sh
-# script/test: run the backend test suite.
+# script/test: run the backend test suite with the race detector and
+# coverage. Go's own -timeout bounds the tests and not their compile,
+# so a cold build cache cannot fail it. The race detector needs cgo,
+# and so a C compiler. If the tests fail, they run again with -v for
+# the details, and the script fails even if that run passes.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    timeout 30 go test ./...
+    go test -timeout 30s -race -cover ./... || {
+        echo "--- Rerunning with -v for details ---"
+        go test -timeout 30s -race -v ./...
+        exit 1
+    }
 }
 
 main "$@"

script/bootstrap

@@ -64,7 +64,8 @@ detect_pkgmgr() {
     fi
 }
 
-# pkg_install <nix-attr> <apt-pkg> <brew-formula> <apk-pkg>
+# pkg_install <nix-attr> <apt-pkgs> <brew-formula> <apk-pkgs>: the apt
+# and apk arguments may each list several packages, separated by spaces.
 pkg_install() {
     detect_pkgmgr
     case "$PKGMGR" in
@@ -74,10 +75,10 @@ pkg_install() {
                 $SUDO env DEBIAN_FRONTEND=noninteractive apt-get update
                 APT_UPDATED=1
             fi
-            $SUDO env DEBIAN_FRONTEND=noninteractive apt-get install -y "$2"
+            $SUDO env DEBIAN_FRONTEND=noninteractive apt-get install -y $2
             ;;
         brew) brew install "$3" ;;
-        apk) apk add --no-cache "$4" ;;
+        apk) apk add --no-cache $4 ;;
     esac
 }
 
@@ -253,6 +254,12 @@ main() {
 
     if missing make; then pkg_install gnumake make make make; fi
     if missing git; then pkg_install git git git git; fi
+    # The race detector in make test needs cgo, which Go turns on only
+    # when it finds its C compiler, gcc on Linux. apt and apk ship the C
+    # library headers apart from gcc.
+    if missing gcc; then
+        pkg_install gcc "gcc libc6-dev" gcc "gcc musl-dev"
+    fi
 
     ensure_node
     ensure_yarn

script/test

@@ -1,14 +1,17 @@
 #!/bin/sh
 # script/test: run the test suite for the whole repo: the frontend at
-# the repo root, then the Go backend in backend/. Both halves together
-# get 30 seconds; each also keeps its own limit for the Dockerfiles.
+# the repo root, then the Go backend in backend/. Each half has its own
+# 30-second limit, and there is none around both: from a cold Go build
+# cache, compiling the backend's tests with the race detector can take
+# 30 seconds on its own, and Go's -timeout leaves the compile out.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    timeout 30 sh -c 'script/frontend-test && backend/script/test'
+    script/frontend-test
+    backend/script/test
 }
 
 main "$@"

src/main.js

@@ -263,7 +263,7 @@ class HostState {
     }
 }
 
-class AppState {
+export class AppState {
     constructor(localHosts) {
         this.wan = WAN_HOSTS.map(
             (h) => new HostState(h, h.name === "datavi.be"),
@@ -1105,7 +1105,7 @@ function sortAndRebuildWAN(state) {
 
 // --- Main Loop ---------------------------------------------------------------
 
-async function tick(state, signal, onOffline) {
+export async function tick(state, signal, onOffline) {
     const ts = Date.now();
 
     if (state.paused) {
@@ -1126,12 +1126,26 @@ async function tick(state, signal, onOffline) {
 
     log.debug(`Tick #${state.tickCount + 1} started`);
 
-    const results = await Promise.all(
-        state.allHosts.map((h) => measureLatency(h.url, signal)),
+    // Each host's row shows its result as soon as its check ends. The
+    // result is discarded if by then the user has paused or the next round
+    // has given up this one's checks, and in the first tick (tickCount is
+    // still 0), which is discarded as a whole below. The row is looked up
+    // when the check ends, as a pin click may have re-sorted the rows since
+    // the round started.
+    await Promise.all(
+        state.allHosts.map(async (host) => {
+            const r = await measureLatency(host.url, signal);
+            if (state.paused || signal.aborted || state.tickCount === 0) {
+                return;
+            }
+            host.pushSample(ts, r);
+            updateHostRow(host, state.allHosts.indexOf(host));
+            log.debug(`${host.name}: ${r.error ? r.error : r.latency + "ms"}`);
+        }),
     );
 
     // User may have paused, or the next round may have given up this
-    // one's checks, while awaiting results — discard them
+    // one's checks, while awaiting results — skip the rest of the round
     if (state.paused || signal.aborted) return;
 
     state.tickCount++;
@@ -1142,13 +1156,6 @@ async function tick(state, signal, onOffline) {
         return;
     }
 
-    state.allHosts.forEach((host, i) => {
-        const r = results[i];
-        host.pushSample(ts, r);
-        updateHostRow(host, i);
-        log.debug(`${host.name}: ${r.error ? r.error : r.latency + "ms"}`);
-    });
-
     // Sort after the first real check, then every 10 ticks thereafter
     if (state.tickCount === 2 || state.tickCount % 10 === 1) {
         sortAndRebuildWAN(state);
@@ -1385,10 +1392,10 @@ async function init() {
     setInterval(updateClocks, 1000);
 
     // Rounds never overlap: a round first gives up the last round's checks
-    // if they are still waiting, and the last round then records nothing.
-    // At a steady interval they never are, as they time out at 80% of it;
-    // they can be when a round starts early, after an interval change or
-    // when the recovery probe finds a target answering.
+    // if they are still waiting, and the last round then records nothing
+    // more. At a steady interval they never are, as they time out at 80% of
+    // it; they can be when a round starts early, after an interval change
+    // or when the recovery probe finds a target answering.
     let roundChecks = new AbortController();
     function doTick() {
         roundChecks.abort();

test/unit/main.test.js

@@ -3,17 +3,45 @@
 
 import { test } from "node:test";
 import assert from "node:assert/strict";
-import { CONFIG, measureLatency } from "../../src/main.js";
+import { AppState, CONFIG, measureLatency, tick } from "../../src/main.js";
 
-// measureLatency writes timeouts to the debug log, which looks for its
-// panel in the page. There is no page here.
-globalThis.document = { getElementById: () => null };
+// There is no page here, so the tests stand in for it. The debug log looks
+// for its panel by id and finds none. Each element of a host's row that
+// tick draws into is a plain object, made the first time it is looked up
+// and kept in elements under its selector. Drawing a sparkline does
+// nothing; it looks for the pixel ratio on window and finds none.
+const elements = {};
+const doNothing = () => {};
+const canvasContext = {
+    clearRect: doNothing,
+    beginPath: doNothing,
+    moveTo: doNothing,
+    lineTo: doNothing,
+    stroke: doNothing,
+    fill: doNothing,
+    fillRect: doNothing,
+    fillText: doNothing,
+    arc: doNothing,
+};
+globalThis.window = {};
+globalThis.document = {
+    getElementById: () => null,
+    querySelector: (selector) =>
+        (elements[selector] ??= { getContext: () => canvasContext }),
+};
+
+// What tick last wrote into the latency figure in host's row, or undefined
+// if it has written nothing there.
+function latencyFigure(state, host) {
+    const index = state.allHosts.indexOf(host);
+    return elements[`.latency-value[data-host="${index}"]`]?.innerHTML;
+}
 
 // Mocks the clock for test t, so that a check lasting seconds takes no real
-// time, and replaces fetch with a target that answers after answerAfter
-// milliseconds of that clock, or never when answerAfter is Infinity. Both
-// are restored when the test ends.
-function mockTarget(t, answerAfter) {
+// time, and replaces fetch with targets that each answer after
+// answerAfter(url) milliseconds of that clock, or never when that is
+// Infinity. Both are restored when the test ends.
+function mockTargets(t, answerAfter) {
     t.mock.timers.enable({ apis: ["setTimeout", "Date"] });
     t.mock.method(performance, "now", () => Date.now());
     t.mock.method(
@@ -21,7 +49,9 @@ function mockTarget(t, answerAfter) {
         "fetch",
         (url, { signal }) =>
             new Promise((resolve, reject) => {
-                if (answerAfter !== Infinity) setTimeout(resolve, answerAfter);
+                if (answerAfter(url) !== Infinity) {
+                    setTimeout(resolve, answerAfter(url));
+                }
                 signal.addEventListener("abort", () => reject(signal.reason));
             }),
     );
@@ -42,7 +72,7 @@ for (const interval of [10000, 30000]) {
 
     test(`at a ${interval}ms interval, an answer after ${slowAnswer}ms is recorded with its real time`, async (t) => {
         CONFIG.updateInterval = interval;
-        mockTarget(t, slowAnswer);
+        mockTargets(t, () => slowAnswer);
         const check = measureLatency("https://target.test");
         t.mock.timers.tick(slowAnswer);
         assert.deepEqual(await settled(check), {
@@ -53,7 +83,7 @@ for (const interval of [10000, 30000]) {
 
     test(`at a ${interval}ms interval, a target that never answers is recorded as a timeout after ${timeout}ms`, async (t) => {
         CONFIG.updateInterval = interval;
-        mockTarget(t, Infinity);
+        mockTargets(t, () => Infinity);
         const check = measureLatency("https://target.test");
         t.mock.timers.tick(timeout - 1);
         assert.equal(await settled(check), "still waiting");
@@ -64,3 +94,29 @@ for (const interval of [10000, 30000]) {
         });
     });
 }
+
+test("at a 30000ms interval, a target answering after 1000ms shows in its row while another target's check is still waiting", async (t) => {
+    CONFIG.updateInterval = 30000;
+    const state = new AppState([
+        { name: "Answering", url: "https://answering.test" },
+    ]);
+    const answering = state.local[0];
+    const waiting = state.wan[0];
+    // No target but the answering one ever answers.
+    mockTargets(t, (url) => (url.startsWith(answering.url) ? 1000 : Infinity));
+    // The third tick: the first is discarded as a whole, and the second ends
+    // by sorting the rows, which rebuilds a page that is not here.
+    state.tickCount = 2;
+
+    const round = tick(state, new AbortController().signal);
+    t.mock.timers.tick(1000);
+    assert.equal(await settled(round), "still waiting");
+    assert.match(latencyFigure(state, answering), />1000</);
+    assert.equal(latencyFigure(state, waiting), undefined);
+    assert.equal(state.tickCount, 2);
+
+    // The round ends, once, when the last check times out.
+    t.mock.timers.tick(CONFIG.requestTimeout - 1000);
+    assert.notEqual(await settled(round), "still waiting");
+    assert.equal(state.tickCount, 3);
+});