fix(课堂看板): 手机上「讲错误」弹层挤成一团，人名和按钮叠在一起；表格横着滑就看不出是谁的格子

- 讲错误弹层在手机上拆成三行：题目和关闭、一排人（横着滑，换到的人自动滑进来）、两个按钮；
  不写「← → 换人」；展开题目时叠在代码上面，不再左右并排
- 错法面板标题行：题名一行，人数挪到第二行，「人」字不再被挤到下一行
- 表格名字列和分组标题钉在左边，横着滑也知道是谁、哪一组

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/src/oj/classroom/board.vue

@@ -615,7 +615,13 @@ function shortTitle(title: string, size: number) {
             </span>
             <span class="muted">没做 {{ problemStats[selectedIndex]!.none }} 人</span>
             <div class="spacer"></div>
-            <n-button quaternary size="small" aria-label="收起" @click="selectedId = null">
+            <n-button
+              quaternary
+              size="small"
+              class="panel-close"
+              aria-label="收起"
+              @click="selectedId = null"
+            >
               <template #icon><Icon icon="ph:x-bold" /></template>
             </n-button>
           </div>
@@ -700,8 +706,10 @@ function shortTitle(title: string, size: number) {
             <template v-for="student in students" :key="student.userId">
               <tr v-if="groupStarts.has(student.userId)" class="group-row">
                 <td :colspan="board.problems.length + 4">
+                  <span class="group-label">
                     {{ GROUP_LABEL[groupOf(student)] }}
                     <span class="muted">{{ groupCount[groupOf(student)] }}</span>
+                  </span>
                 </td>
               </tr>
               <tr :class="{ 'leave-row': groupOf(student) === 'leave' }">
@@ -1164,6 +1172,14 @@ a.who {
   color: inherit;
 }
 
+/* 题多了表格要横着滑，名字钉在左边，不然滑过去就不知道是谁的格子 */
+.grid .name {
+  position: sticky;
+  left: 0;
+  z-index: 1;
+  background: v-bind("theme.cardColor");
+}
+
 .grid th.col {
   width: 96px;
   cursor: pointer;
@@ -1213,6 +1229,12 @@ a.who {
   background: v-bind("theme.actionColor");
 }
 
+/* 和名字一样钉在左边：横着滑过去也看得见这一组是什么 */
+.group-label {
+  position: sticky;
+  left: 12px;
+}
+
 .group-row .muted {
   margin-left: 4px;
   font-weight: 400;
@@ -1314,4 +1336,57 @@ a.cell {
 .meta {
   font-size: var(--oj-fs-sec);
 }
+
+/* ---------- 手机 ---------- */
+
+@media (max-width: 768px) {
+  /* 题名占一行（收起钉在右边），人数挪到第二行，别把「人」挤到下一行去 */
+  .panel-head {
+    flex-wrap: wrap;
+    gap: 6px 12px;
+  }
+
+  .panel-head > span {
+    order: 2;
+    white-space: nowrap;
+  }
+
+  .panel-title {
+    flex: 1 1 calc(100% - 50px);
+    min-width: 0;
+  }
+
+  .panel-head .spacer {
+    display: none;
+  }
+
+  .panel-close {
+    order: 1;
+  }
+
+  .err-group {
+    min-width: 0;
+    flex: 1 1 100%;
+    box-sizing: border-box;
+  }
+
+  .start-row {
+    flex-wrap: wrap;
+  }
+
+  .start-input {
+    flex-basis: 100%;
+    order: -1;
+  }
+
+  .grid .name {
+    width: auto;
+    padding-left: 4px;
+    padding-right: 8px;
+  }
+
+  .cell {
+    width: 48px;
+  }
+}
 </style>

apps/web/src/oj/classroom/components/ExplainModal.vue

@@ -14,6 +14,7 @@ import { onKeyStroke } from "@vueuse/core"
 import { useThemeVars } from "naive-ui"
 import SubmissionView from "oj/submission/components/SubmissionView.vue"
 import UserName from "shared/components/UserName.vue"
+import { useBreakpoints } from "shared/composables/breakpoints"
 import type { ClassBoardProblem } from "utils/types"
 import ProblemBrief from "./ProblemBrief.vue"
 
@@ -24,6 +25,9 @@ import ProblemBrief from "./ProblemBrief.vue"
  *
  * 「题目」按钮：演示错误时题目和代码要并排（用户要的），点了左边出题目，再点收起。
  * 平时收着，代码照样占满宽。开没开记在这台电脑上，讲下一个人、下一道题还是那样
+ *
+ * 手机上（老师拿手机看看板）：顶上拆成三行 —— 题目和关闭、一排人（横着滑）、两个按钮；
+ * 没有键盘所以不写「← → 换人」；题目展开时叠在代码上面，不并排
  */
 const props = defineProps<{
   problem: ClassBoardProblem | null
@@ -33,6 +37,7 @@ const show = defineModel<boolean>("show", { required: true })
 const index = defineModel<number>("index", { required: true })
 
 const theme = useThemeVars()
+const { isMobile } = useBreakpoints()
 /**
  * 颜色走内联的 CSS 变量，不用 <style> 里的 v-bind()：n-modal 把内容挂到 body 下面，
  * v-bind 的变量挂在组件根上，传不过去（弹层底色整个透明）
@@ -50,6 +55,16 @@ const vars = computed(() => ({
 const current = computed(() => props.people[index.value] ?? null)
 const showProblem = useStorage("oj_explain_show_problem", false)
 
+/** 一排人横着滑的时候，换到的人要滑进来（← → 换人、或者点开的就是排在后面的） */
+const peopleRef = ref<HTMLElement | null>(null)
+watch([index, show], async () => {
+  if (!show.value) return
+  await nextTick()
+  peopleRef.value
+    ?.querySelector<HTMLElement>(".person.on")
+    ?.scrollIntoView({ block: "nearest", inline: "nearest" })
+})
+
 function step(delta: -1 | 1) {
   const next = index.value + delta
   if (next >= 0 && next < props.people.length) index.value = next
@@ -68,6 +83,7 @@ onKeyStroke(["ArrowLeft", "ArrowRight"], (event) => {
     <!-- 无 preset 的 n-modal 直接子节点要是普通元素 -->
     <div
       class="sheet"
+      :class="{ narrow: isMobile }"
       :style="vars"
       role="dialog"
       :aria-label="problem ? `讲 ${problem.problemDisplayId} 的错误` : '讲错误'"
@@ -77,7 +93,7 @@ onKeyStroke(["ArrowLeft", "ArrowRight"], (event) => {
           <b>{{ problem.problemDisplayId }}</b> {{ problem.title }}
           <template v-if="people.length > 1"> · 错着的</template>
         </span>
-        <div class="people">
+        <div ref="peopleRef" class="people">
           <button
             v-for="(person, i) in people"
             :key="person.userId"
@@ -90,10 +106,14 @@ onKeyStroke(["ArrowLeft", "ArrowRight"], (event) => {
             <span class="kind">{{ person.label }}</span>
           </button>
         </div>
-        <span v-if="people.length > 1" class="keys"><kbd>←</kbd><kbd>→</kbd> 换人</span>
+        <span v-if="people.length > 1 && !isMobile" class="keys"
+          ><kbd>←</kbd><kbd>→</kbd> 换人</span
+        >
         <div class="spacer"></div>
+        <div class="buttons">
           <n-button
             v-if="problem"
+            :size="isMobile ? 'small' : 'medium'"
             :type="showProblem ? 'primary' : 'default'"
             :secondary="showProblem"
             @click="showProblem = !showProblem"
@@ -102,13 +122,15 @@ onKeyStroke(["ArrowLeft", "ArrowRight"], (event) => {
           </n-button>
           <n-button
             v-if="current"
+            :size="isMobile ? 'small' : 'medium'"
             tag="a"
             :href="'/submission/' + current.submissionId"
             target="_blank"
           >
             在新页面打开
           </n-button>
-        <n-button quaternary aria-label="关闭" @click="show = false">
+        </div>
+        <n-button quaternary class="close" aria-label="关闭" @click="show = false">
           <template #icon><Icon icon="ph:x-bold" /></template>
         </n-button>
       </div>
@@ -238,4 +260,70 @@ kbd {
 .main {
   min-width: 0;
 }
+
+.buttons {
+  display: flex;
+  gap: 10px;
+}
+
+/* ---------- 手机 ---------- */
+
+.sheet.narrow {
+  width: calc(100vw - 16px);
+  max-height: calc(100vh - 16px);
+  padding: 12px 14px 16px;
+  gap: 12px;
+}
+
+/* 第一行题目 + 关闭，第二行一排人（横着滑），第三行两个按钮 */
+.narrow .top {
+  display: grid;
+  grid-template-columns: minmax(0, 1fr) auto;
+  gap: 10px;
+}
+
+.narrow .problem {
+  grid-row: 1;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.narrow .close {
+  grid-row: 1;
+  grid-column: 2;
+  margin-right: -6px;
+}
+
+.narrow .people {
+  grid-column: 1 / -1;
+  flex-wrap: nowrap;
+  overflow-x: auto;
+  scrollbar-width: none;
+}
+
+.narrow .person {
+  flex-shrink: 0;
+}
+
+.narrow .spacer {
+  display: none;
+}
+
+.narrow .buttons {
+  grid-column: 1 / -1;
+}
+
+/* 题目叠在代码上面，自己滚，别把代码顶出一屏 */
+.narrow .body.split {
+  display: flex;
+  flex-direction: column;
+  gap: 14px;
+}
+
+.narrow .statement {
+  position: static;
+  max-height: 40vh;
+  padding: 0 0 12px;
+  border-bottom: 1px solid var(--divider);
+}
 </style>