diff --git a/apps/web/src/oj/classroom/board.vue b/apps/web/src/oj/classroom/board.vue
index d259495..999174c 100644
--- a/apps/web/src/oj/classroom/board.vue
+++ b/apps/web/src/oj/classroom/board.vue
@@ -16,12 +16,14 @@ import type { ClassBoard, ClassBoardCell, ClassBoardProblem, ClassBoardStudent }
 import ExplainModal, { type ExplainPerson } from "./components/ExplainModal.vue"
 import LeaveModal from "./components/LeaveModal.vue"
 import LessonForm from "./components/LessonForm.vue"
+import TrailPanel from "./components/TrailPanel.vue"
 import { errorKind, type ErrorKind } from "utils/errorKind"
 import {
   cellStyle as cellStyleOf,
   cellText,
   cellTitle as cellTitleOf,
   STUCK_ATTEMPTS,
+  trailSummary,
 } from "./boardCells"
 
 /**
@@ -378,6 +380,22 @@ const cellStyle = (cell: ClassBoardCell) => cellStyleOf(cell, board.value?.day,
 const cellTitle = (cell: ClassBoardCell, student: ClassBoardStudent) =>
   cellTitleOf(cell, student, board.value?.day)
 
+/**
+ * 点开着轨迹的那个格子（学生 id:第几道题）。做对了、今天交过代码的格子点了不再直接去提交列表，
+ * 先在格子下面看他是怎么一次次做对的，列表在弹出来的那块里
+ */
+const trailKey = ref<string | null>(null)
+
+function toggleTrail(key: string) {
+  trailKey.value = trailKey.value === key ? null : key
+}
+
+/** 点到别的格子时，那边自己会换过去；这里只管点到空白处收起 */
+function trailOutside(event: MouseEvent) {
+  if ((event.target as Element | null)?.closest(".cell")) return
+  trailKey.value = null
+}
+
 /**
  * 点格子 / 名字到提交列表看他今天交的。新标签打开：看板一般开着不关，别把它顶掉。
  * 用户名按整名匹配（学号互相包含是常态，ks24a1 会混进 ks24a10）。
@@ -520,10 +538,19 @@ function lastText(student: ClassBoardStudent) {
 
         <section v-if="selectedIndex >= 0" class="panel">
           <div class="panel-head">
-            <b class="panel-title">
+            <a
+              class="panel-title"
+              :href="
+                router.resolve({
+                  name: 'problem',
+                  params: { problemID: board.problems[selectedIndex]!.problemDisplayId },
+                }).href
+              "
+              target="_blank"
+            >
               <span class="muted">{{ board.problems[selectedIndex]!.problemDisplayId }}</span>
               {{ board.problems[selectedIndex]!.title }}
-            </b>
+            </a>
             <span>
               错着的 <b class="wrong">{{ problemStats[selectedIndex]!.wrong }}</b> 人
             </span>
@@ -688,6 +715,35 @@ function lastText(student: ClassBoardStudent) {
                   >
                     {{ cellText(cell) }}
                   </button>
+                  <n-popover
+                    v-else-if="cell.trail.length"
+                    :show="trailKey === `${student.userId}:${i}`"
+                    trigger="manual"
+                    placement="bottom-start"
+                    :style="{ width: '620px', maxWidth: 'calc(100vw - 32px)' }"
+                    @clickoutside="trailOutside"
+                  >
+                    <template #trigger>
+                      <button
+                        type="button"
+                        class="cell"
+                        :class="{ picked: trailKey === `${student.userId}:${i}` }"
+                        :style="cellStyle(cell)"
+                        :title="cellTitle(cell, student)"
+                        @click="toggleTrail(`${student.userId}:${i}`)"
+                      >
+                        {{ cellText(cell) }}
+                      </button>
+                    </template>
+                    <TrailPanel
+                      :username="student.username"
+                      :problem="board.problems[i]!"
+                      :cell="cell"
+                      :summary="trailSummary(cell, board.day)"
+                      :list-href="submissionsHref(student, board.problems[i], false)"
+                      @close="trailKey = null"
+                    />
+                  </n-popover>
                   <a
                     v-else-if="cell.attempts"
                     class="cell"
@@ -941,6 +997,13 @@ function lastText(student: ClassBoardStudent) {
 
 .panel-title {
   font-size: 16px;
+  font-weight: 600;
+  color: inherit;
+  text-decoration: none;
+}
+
+.panel-title:hover {
+  color: v-bind("theme.primaryColor");
 }
 
 .groups {
@@ -1157,6 +1220,10 @@ a.cell {
   cursor: pointer;
 }
 
+.cell.picked {
+  box-shadow: 0 0 0 2px v-bind("theme.textColor1");
+}
+
 .num {
   font-variant-numeric: tabular-nums;
 }
diff --git a/apps/web/src/oj/classroom/boardCells.ts b/apps/web/src/oj/classroom/boardCells.ts
index 2525db1..c81172e 100644
--- a/apps/web/src/oj/classroom/boardCells.ts
+++ b/apps/web/src/oj/classroom/boardCells.ts
@@ -15,8 +15,9 @@ export function acceptedToday(cell: ClassBoardCell, day: string | undefined) {
   return !!cell.acceptedAt && parseTime(cell.acceptedAt, "YYYY-MM-DD") === day
 }
 
+/** 做对了的写第几次做对：「✓ 9」是今天第 9 次才对，一次就对的、以前就对了的还是「✓」 */
 export function cellText(cell: ClassBoardCell) {
-  if (cell.status === "accepted") return "✓"
+  if (cell.status === "accepted") return (cell.acceptedTry ?? 1) > 1 ? `✓ ${cell.acceptedTry}` : "✓"
   if (cell.status === "tried") return cell.attempts ? `✗ ${cell.attempts}` : "✗"
   return ""
 }
@@ -62,3 +63,13 @@ export function cellTitle(
   }
   return ""
 }
+
+/** 点开做对的格子，方块串旁边那句 */
+export function trailSummary(cell: ClassBoardCell, day: string | undefined) {
+  const total = cell.trail.length
+  if (cell.acceptedTry === 1) return total === 1 ? "一次就做对" : `交了 ${total} 次，第 1 次就做对`
+  if (cell.acceptedTry) return `交了 ${total} 次，第 ${cell.acceptedTry} 次做对`
+  return acceptedToday(cell, day)
+    ? `今天交了 ${total} 次代码`
+    : `以前就做对了，今天又交了 ${total} 次`
+}
diff --git a/apps/web/src/oj/classroom/components/TrailPanel.vue b/apps/web/src/oj/classroom/components/TrailPanel.vue
new file mode 100644
index 0000000..47df5d8
--- /dev/null
+++ b/apps/web/src/oj/classroom/components/TrailPanel.vue
@@ -0,0 +1,228 @@
+<script setup lang="ts">
+import { Icon } from "@iconify/vue"
+import { useThemeVars } from "naive-ui"
+import { useTone } from "shared/composables/tone"
+import UserName from "shared/components/UserName.vue"
+import { attemptTone, cachedSubmission } from "oj/statistics/attempts"
+import { JUDGE_STATUS, LANGUAGE_FORMAT_VALUE } from "utils/constants"
+import { parseTime } from "utils/functions"
+import type { ClassBoardCell, ClassBoardProblem, Submission } from "utils/types"
+
+/**
+ * 点开一个做对了的格子：他这道题今天交的每一次，一格一次、从早到晚，下面是停着的那一次的代码
+ * （设计稿「课堂看板重设计」· 做对的格子也看得到轨迹）。
+ *
+ * 默认停在做对前最后错的那一次 —— 要讲的一般是「他最后是改了哪儿才对的」。
+ * 停上去就换，不用点：老师在几格之间来回比；点也行（手机上没有悬停）
+ */
+const props = defineProps<{
+  username: string
+  problem: ClassBoardProblem
+  cell: ClassBoardCell
+  summary: string
+  /** 「在提交列表看这 N 次」：和格子原来点进去的是同一个列表 */
+  listHref: string
+}>()
+defineEmits<{ close: [] }>()
+
+const tone = useTone()
+const theme = useThemeVars()
+
+const picked = ref(
+  props.cell.acceptedTry
+    ? Math.max(props.cell.acceptedTry - 2, 0)
+    : Math.max(props.cell.trail.length - 1, 0),
+)
+const item = computed(() => props.cell.trail[picked.value] ?? props.cell.trail.at(-1))
+
+const code = reactive<Record<string, Submission | "loading" | "failed">>({})
+watch(
+  () => item.value?.id,
+  async (id) => {
+    if (!id || (code[id] && code[id] !== "failed")) return
+    code[id] = "loading"
+    try {
+      code[id] = await cachedSubmission(id)
+    } catch {
+      code[id] = "failed"
+    }
+  },
+  { immediate: true },
+)
+const shown = computed(() => {
+  const value = item.value && code[item.value.id]
+  return typeof value === "object" ? value : null
+})
+
+const squares = computed(() =>
+  props.cell.trail.map((row) => ({
+    ...row,
+    color: tone(attemptTone(row.result)).solid,
+  })),
+)
+</script>
+
+<template>
+  <div class="trail">
+    <div class="head">
+      <UserName :username="username" />
+      <span class="problem">
+        <b>{{ problem.problemDisplayId }}</b>
+        <span class="muted">{{ problem.title }}</span>
+      </span>
+      <div class="spacer"></div>
+      <n-button quaternary size="tiny" aria-label="收起" @click="$emit('close')">
+        <template #icon><Icon icon="ph:x-bold" /></template>
+      </n-button>
+    </div>
+
+    <div class="line">
+      <div class="squares">
+        <button
+          v-for="(row, index) in squares"
+          :key="row.id"
+          type="button"
+          class="square"
+          :class="{ on: index === picked }"
+          :style="{ background: row.color }"
+          :aria-label="`第 ${index + 1} 次：${JUDGE_STATUS[row.result]?.name ?? ''}`"
+          @mouseenter="picked = index"
+          @click="picked = index"
+        ></button>
+      </div>
+      <span class="summary">{{ summary }}</span>
+    </div>
+
+    <div v-if="item" class="code">
+      <div class="code-head">
+        <b :style="{ color: tone(attemptTone(item.result)).color }">
+          {{ JUDGE_STATUS[item.result]?.name }}
+        </b>
+        <span class="muted"
+          >{{ parseTime(item.createTime, "HH:mm") }} · 第 {{ picked + 1 }} 次</span
+        >
+      </div>
+      <n-code
+        v-if="shown"
+        class="code-body"
+        :code="shown.code"
+        :language="LANGUAGE_FORMAT_VALUE[shown.language]"
+        show-line-numbers
+      />
+      <span v-else-if="code[item.id] === 'failed'" class="muted">代码没拉下来，点下面打开看</span>
+      <span v-else class="muted">代码加载中…</span>
+    </div>
+
+    <div class="links">
+      <a v-if="item" :href="`/submission/${item.id}`" target="_blank">打开这次提交 ↗</a>
+      <a :href="listHref" target="_blank">在提交列表看这 {{ cell.trail.length }} 次 ↗</a>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.trail {
+  display: flex;
+  flex-direction: column;
+  gap: 12px;
+  font-size: 14px;
+}
+
+.head {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  min-width: 0;
+  font-size: 15px;
+  white-space: nowrap;
+}
+
+.problem {
+  display: flex;
+  gap: 6px;
+  min-width: 0;
+  font-size: 14px;
+}
+
+.problem .muted {
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.spacer {
+  flex: 1;
+}
+
+.line {
+  display: flex;
+  align-items: center;
+  gap: 14px;
+}
+
+.squares {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 6px;
+}
+
+.square {
+  flex: none;
+  width: 22px;
+  height: 22px;
+  padding: 0;
+  border: 0;
+  border-radius: 4px;
+  cursor: pointer;
+}
+
+.square.on {
+  box-shadow:
+    0 0 0 2px v-bind("theme.cardColor"),
+    0 0 0 4px v-bind("theme.textColor1");
+}
+
+.summary {
+  white-space: nowrap;
+  font-variant-numeric: tabular-nums;
+  color: v-bind("theme.textColor2");
+}
+
+.code {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+  padding: 10px 12px;
+  border-radius: 6px;
+  background: v-bind("theme.actionColor");
+}
+
+.code-head {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+}
+
+.code-body {
+  max-height: 320px;
+  overflow: auto;
+  font-size: 15px;
+}
+
+.links {
+  display: flex;
+  gap: 18px;
+}
+
+.links a {
+  color: v-bind("theme.primaryColor");
+  text-decoration: none;
+}
+
+.links a:hover {
+  text-decoration: underline;
+}
+
+.muted {
+  color: v-bind("theme.textColor3");
+}
+</style>
diff --git a/apps/web/src/oj/statistics/attempts.ts b/apps/web/src/oj/statistics/attempts.ts
new file mode 100644
index 0000000..709f0e2
--- /dev/null
+++ b/apps/web/src/oj/statistics/attempts.ts
@@ -0,0 +1,40 @@
+import { getSubmission } from "oj/api"
+import { SubmissionStatus } from "utils/constants"
+import type { Submission } from "utils/types"
+import type { Tone } from "shared/composables/tone"
+
+/**
+ * 一次提交一格的那串方块（统计页的 AttemptSquares、课堂看板做对格子的轨迹）共用的两样：
+ * 方块的颜色，和停上去看代码时拉的提交。
+ */
+
+/** 对了绿；编译失败、运行时错误、语法规则没过、部分通过橙；判题中蓝；其余红 */
+export function attemptTone(result: number): Tone {
+  if (result === SubmissionStatus.accepted) return "success"
+  if (
+    result === SubmissionStatus.compile_error ||
+    result === SubmissionStatus.runtime_error ||
+    result === SubmissionStatus.ast_check_failed ||
+    result === SubmissionStatus.partial_accepted
+  ) {
+    return "warning"
+  }
+  if (result === SubmissionStatus.pending || result === SubmissionStatus.judging) return "info"
+  return "error"
+}
+
+/**
+ * 代码按提交 id 缓存，整页共用：交了就不会再变，老师在一排方块之间来回对比时
+ * 不用每次都等网络。失败的不进缓存，下次停上去再试。
+ */
+const codeCache = new Map<string, Promise<Submission>>()
+
+export function cachedSubmission(id: string) {
+  let hit = codeCache.get(id)
+  if (!hit) {
+    hit = getSubmission(id)
+    hit.catch(() => codeCache.delete(id))
+    codeCache.set(id, hit)
+  }
+  return hit
+}
diff --git a/apps/web/src/oj/statistics/components/AttemptSquares.vue b/apps/web/src/oj/statistics/components/AttemptSquares.vue
index b8c20e8..2fcb14d 100644
--- a/apps/web/src/oj/statistics/components/AttemptSquares.vue
+++ b/apps/web/src/oj/statistics/components/AttemptSquares.vue
@@ -1,20 +1,10 @@
-<script lang="ts">
-import type { Submission } from "utils/types"
-
-/**
- * 代码按提交 id 缓存，整页共用：交了就不会再变，老师在一排方块之间来回对比时
- * 不用每次都等网络。失败的不进缓存，下次停上去再试。
- */
-const codeCache = new Map<string, Submission>()
-</script>
-
 <script setup lang="ts">
 import { useThemeVars } from "naive-ui"
-import { getSubmission } from "oj/api"
-import { JUDGE_STATUS, LANGUAGE_FORMAT_VALUE, SubmissionStatus } from "utils/constants"
+import { JUDGE_STATUS, LANGUAGE_FORMAT_VALUE } from "utils/constants"
 import { parseTime } from "utils/functions"
-import type { SUBMISSION_RESULT } from "utils/types"
-import { useTone, type Tone } from "shared/composables/tone"
+import type { Submission, SUBMISSION_RESULT } from "utils/types"
+import { useTone } from "shared/composables/tone"
+import { attemptTone, cachedSubmission } from "../attempts"
 
 /**
  * 一个人在一道题上的每一次提交，一格一次、从早到晚（设计稿「统计重设计」）。
@@ -31,34 +21,13 @@ const props = withDefaults(
 const tone = useTone()
 const theme = useThemeVars()
 
-function toneOf(result: number): Tone {
-  if (result === SubmissionStatus.accepted) return "success"
-  if (
-    result === SubmissionStatus.compile_error ||
-    result === SubmissionStatus.runtime_error ||
-    result === SubmissionStatus.ast_check_failed ||
-    result === SubmissionStatus.partial_accepted
-  ) {
-    return "warning"
-  }
-  if (result === SubmissionStatus.pending || result === SubmissionStatus.judging) return "info"
-  return "error"
-}
-
 const code = reactive<Record<string, Submission | "loading" | "failed">>({})
 
 async function load(id: string) {
-  const hit = codeCache.get(id)
-  if (hit) {
-    code[id] = hit
-    return
-  }
-  if (code[id] === "loading") return
+  if (code[id] && code[id] !== "failed") return
   code[id] = "loading"
   try {
-    const res = await getSubmission(id)
-    codeCache.set(id, res)
-    code[id] = res
+    code[id] = await cachedSubmission(id)
   } catch {
     code[id] = "failed"
   }
@@ -72,7 +41,7 @@ const cells = computed(() =>
   props.items.map((item, index) => ({
     ...item,
     index: index + 1,
-    color: tone(toneOf(item.result)).solid,
+    color: tone(attemptTone(item.result)).solid,
   })),
 )
 </script>