diff --git a/frontend/taskdeck-web/src/components/board/BoardCanvas.vue b/frontend/taskdeck-web/src/components/board/BoardCanvas.vue
index 0fb0b7d92..5a26d7db2 100644
--- a/frontend/taskdeck-web/src/components/board/BoardCanvas.vue
+++ b/frontend/taskdeck-web/src/components/board/BoardCanvas.vue
@@ -14,7 +14,7 @@ defineProps<{
   selectedCardId: string | null
 }>()
 
-defineEmits<{
+const emit = defineEmits<{
   columnDragStart: [column: Column, event: DragEvent]
   columnDragEnd: []
   columnDragOver: [column: Column, event: DragEvent]
@@ -24,6 +24,23 @@ defineEmits<{
   cardDragEnd: []
   cardEditorSavingChange: [saving: boolean]
 }>()
+
+// One CardModal lives inside each ColumnLane. The route boundary needs the
+// aggregate, not whichever lane happened to emit last: a clean modal closing
+// in column B must not clear column A's unanswered assignment PUT.
+const savingColumnIds = new Set<string>()
+let aggregateCardEditorSaving = false
+
+function handleCardEditorSavingChange(columnId: string, saving: boolean) {
+  if (saving) savingColumnIds.add(columnId)
+  else savingColumnIds.delete(columnId)
+
+  const nextAggregate = savingColumnIds.size > 0
+  if (nextAggregate === aggregateCardEditorSaving) return
+
+  aggregateCardEditorSaving = nextAggregate
+  emit('cardEditorSavingChange', nextAggregate)
+}
 </script>
 
 <template>
@@ -58,7 +75,7 @@ defineEmits<{
           :selected-card-id="selectedCardId"
           @card-drag-start="$emit('cardDragStart', $event)"
           @card-drag-end="$emit('cardDragEnd')"
-          @card-editor-saving-change="$emit('cardEditorSavingChange', $event)"
+          @card-editor-saving-change="handleCardEditorSavingChange(column.id, $event)"
         />
       </div>
 
diff --git a/frontend/taskdeck-web/src/tests/components/BoardCanvasSaveOwnership.spec.ts b/frontend/taskdeck-web/src/tests/components/BoardCanvasSaveOwnership.spec.ts
new file mode 100644
index 000000000..70023ec66
--- /dev/null
+++ b/frontend/taskdeck-web/src/tests/components/BoardCanvasSaveOwnership.spec.ts
@@ -0,0 +1,95 @@
+import { describe, expect, it } from 'vitest'
+import { mount } from '@vue/test-utils'
+import BoardCanvas from '../../components/board/BoardCanvas.vue'
+import type { Column } from '../../types/board'
+
+const columns: Column[] = [
+  {
+    id: 'column-a',
+    boardId: 'board-1',
+    name: 'Todo',
+    position: 0,
+    wipLimit: null,
+    cardCount: 0,
+    createdAt: '2026-09-20T10:00:00Z',
+    updatedAt: '2026-09-20T10:00:00Z',
+  },
+  {
+    id: 'column-b',
+    boardId: 'board-1',
+    name: 'Done',
+    position: 1,
+    wipLimit: null,
+    cardCount: 0,
+    createdAt: '2026-09-20T10:00:00Z',
+    updatedAt: '2026-09-20T10:00:00Z',
+  },
+]
+
+function mountCanvas() {
+  return mount(BoardCanvas, {
+    props: {
+      sortedColumns: columns,
+      cardsByColumn: new Map(),
+      labels: [],
+      boardId: 'board-1',
+      hasColumns: true,
+      draggedColumn: null,
+      dragOverColumnId: null,
+      draggedCard: null,
+      selectedCardId: null,
+    },
+    global: {
+      stubs: {
+        ColumnLane: {
+          name: 'ColumnLane',
+          props: ['column'],
+          emits: ['card-editor-saving-change'],
+          template: `
+            <section :data-testid="'lane-' + column.id">
+              <button
+                :data-testid="'start-' + column.id"
+                @click="$emit('card-editor-saving-change', true)"
+              >start</button>
+              <button
+                :data-testid="'settle-' + column.id"
+                @click="$emit('card-editor-saving-change', false)"
+              >settle</button>
+            </section>
+          `,
+        },
+      },
+    },
+  })
+}
+
+describe('BoardCanvas assignment-save ownership', () => {
+  it('stays globally saving until the final lane owner settles', async () => {
+    const wrapper = mountCanvas()
+
+    await wrapper.get('[data-testid="start-column-a"]').trigger('click')
+    expect(wrapper.emitted('cardEditorSavingChange')).toEqual([[true]])
+
+    await wrapper.get('[data-testid="start-column-b"]').trigger('click')
+    expect(wrapper.emitted('cardEditorSavingChange')).toEqual([[true]])
+
+    await wrapper.get('[data-testid="settle-column-a"]').trigger('click')
+    expect(wrapper.emitted('cardEditorSavingChange')).toEqual([[true]])
+
+    await wrapper.get('[data-testid="settle-column-b"]').trigger('click')
+    expect(wrapper.emitted('cardEditorSavingChange')).toEqual([[true], [false]])
+  })
+
+  it('treats duplicate lane events as idempotent aggregate state', async () => {
+    const wrapper = mountCanvas()
+
+    await wrapper.get('[data-testid="start-column-a"]').trigger('click')
+    await wrapper.get('[data-testid="start-column-a"]').trigger('click')
+    await wrapper.get('[data-testid="settle-column-b"]').trigger('click')
+    expect(wrapper.emitted('cardEditorSavingChange')).toEqual([[true]])
+
+    await wrapper.get('[data-testid="settle-column-a"]').trigger('click')
+    await wrapper.get('[data-testid="settle-column-a"]').trigger('click')
+    expect(wrapper.emitted('cardEditorSavingChange')).toEqual([[true], [false]])
+  })
+})