diff --git a/src/renderer/src/components/tab-group/TabGroupSplitLayout.tsx b/src/renderer/src/components/tab-group/TabGroupSplitLayout.tsx
index db37a19d241..b782751742a 100644
--- a/src/renderer/src/components/tab-group/TabGroupSplitLayout.tsx
+++ b/src/renderer/src/components/tab-group/TabGroupSplitLayout.tsx
@@ -6,6 +6,7 @@ import TabGroupPanel from './TabGroupPanel'
 import TabDragPreview from '../tab-bar/TabDragPreview'
 import { TabDragProvider } from './tab-drag-context'
 import TabPaneColumnSplitDragOverlay from './TabPaneColumnSplitDragOverlay'
+import { useRefitOnSplitCollapse } from './use-refit-on-split-collapse'
 import { type HoveredTabInsertion, useTabDragSplit } from './useTabDragSplit'
 
 const MIN_RATIO = 0.15
@@ -223,6 +224,9 @@ export default function TabGroupSplitLayout({
   const dragSplit = useTabDragSplit({ worktreeId, enabled: isWorktreeActive })
   const hasSplits = layout.type === 'split'
 
+  // Why: reflow the surviving group's terminal when a split collapses (#6154).
+  useRefitOnSplitCollapse(layout, isWorktreeActive)
+
   return (
     <TabDragProvider
       isTabDragActive={dragSplit.activeDrag !== null}
diff --git a/src/renderer/src/components/tab-group/tab-group-layout-leaf-count.test.ts b/src/renderer/src/components/tab-group/tab-group-layout-leaf-count.test.ts
new file mode 100644
index 00000000000..7f24cdeb231
--- /dev/null
+++ b/src/renderer/src/components/tab-group/tab-group-layout-leaf-count.test.ts
@@ -0,0 +1,40 @@
+import { describe, expect, it } from 'vitest'
+import type { TabGroupLayoutNode } from '../../../../shared/types'
+import { countLayoutLeaves } from './tab-group-layout-leaf-count'
+
+describe('countLayoutLeaves', () => {
+  it('counts a single leaf as one', () => {
+    const layout: TabGroupLayoutNode = { type: 'leaf', groupId: 'a' }
+    expect(countLayoutLeaves(layout)).toBe(1)
+  })
+
+  it('counts a two-leaf split as two', () => {
+    const layout: TabGroupLayoutNode = {
+      type: 'split',
+      direction: 'horizontal',
+      first: { type: 'leaf', groupId: 'a' },
+      second: { type: 'leaf', groupId: 'b' }
+    }
+    expect(countLayoutLeaves(layout)).toBe(2)
+  })
+
+  it('counts leaves in a nested split tree', () => {
+    const layout: TabGroupLayoutNode = {
+      type: 'split',
+      direction: 'horizontal',
+      first: { type: 'leaf', groupId: 'a' },
+      second: {
+        type: 'split',
+        direction: 'vertical',
+        first: { type: 'leaf', groupId: 'b' },
+        second: {
+          type: 'split',
+          direction: 'horizontal',
+          first: { type: 'leaf', groupId: 'c' },
+          second: { type: 'leaf', groupId: 'd' }
+        }
+      }
+    }
+    expect(countLayoutLeaves(layout)).toBe(4)
+  })
+})
diff --git a/src/renderer/src/components/tab-group/tab-group-layout-leaf-count.ts b/src/renderer/src/components/tab-group/tab-group-layout-leaf-count.ts
new file mode 100644
index 00000000000..22c3b21531d
--- /dev/null
+++ b/src/renderer/src/components/tab-group/tab-group-layout-leaf-count.ts
@@ -0,0 +1,9 @@
+import type { TabGroupLayoutNode } from '../../../../shared/types'
+
+/** Total number of group leaves in a tab-group split tree (leaf → 1, split → sum of children). */
+export function countLayoutLeaves(node: TabGroupLayoutNode): number {
+  if (node.type === 'leaf') {
+    return 1
+  }
+  return countLayoutLeaves(node.first) + countLayoutLeaves(node.second)
+}
diff --git a/src/renderer/src/components/tab-group/use-refit-on-split-collapse.test.tsx b/src/renderer/src/components/tab-group/use-refit-on-split-collapse.test.tsx
new file mode 100644
index 00000000000..62562a9c66f
--- /dev/null
+++ b/src/renderer/src/components/tab-group/use-refit-on-split-collapse.test.tsx
@@ -0,0 +1,130 @@
+// @vitest-environment happy-dom
+
+import { act } from 'react'
+import { createRoot, type Root } from 'react-dom/client'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import type { TabGroupLayoutNode } from '../../../../shared/types'
+import { SYNC_FIT_PANES_EVENT } from '@/constants/terminal'
+import { useRefitOnSplitCollapse } from './use-refit-on-split-collapse'
+
+const LEAF: TabGroupLayoutNode = { type: 'leaf', groupId: 'a' }
+const SPLIT_2: TabGroupLayoutNode = {
+  type: 'split',
+  direction: 'horizontal',
+  first: { type: 'leaf', groupId: 'a' },
+  second: { type: 'leaf', groupId: 'b' }
+}
+const SPLIT_3: TabGroupLayoutNode = {
+  type: 'split',
+  direction: 'horizontal',
+  first: { type: 'leaf', groupId: 'a' },
+  second: {
+    type: 'split',
+    direction: 'vertical',
+    first: { type: 'leaf', groupId: 'b' },
+    second: { type: 'leaf', groupId: 'c' }
+  }
+}
+
+function Probe({
+  layout,
+  isWorktreeActive
+}: {
+  layout: TabGroupLayoutNode
+  isWorktreeActive: boolean
+}): null {
+  useRefitOnSplitCollapse(layout, isWorktreeActive)
+  return null
+}
+
+describe('useRefitOnSplitCollapse', () => {
+  let container: HTMLDivElement
+  let root: Root
+  let dispatched: number
+  let onSyncFit: () => void
+  // Two-deep rAF callback queue so the double-rAF resolves deterministically.
+  let rafCallbacks: FrameRequestCallback[]
+
+  const flushRaf = (): void => {
+    // Drain in waves so a callback scheduled by another callback runs next tick.
+    let guard = 0
+    while (rafCallbacks.length > 0 && guard < 10) {
+      const batch = rafCallbacks
+      rafCallbacks = []
+      for (const cb of batch) {
+        cb(performance.now())
+      }
+      guard += 1
+    }
+  }
+
+  const renderLayout = (layout: TabGroupLayoutNode, isWorktreeActive = true): void => {
+    act(() => {
+      root.render(<Probe layout={layout} isWorktreeActive={isWorktreeActive} />)
+    })
+  }
+
+  beforeEach(() => {
+    container = document.createElement('div')
+    document.body.appendChild(container)
+    root = createRoot(container)
+    dispatched = 0
+    rafCallbacks = []
+    onSyncFit = (): void => {
+      dispatched += 1
+    }
+    window.addEventListener(SYNC_FIT_PANES_EVENT, onSyncFit)
+    vi.stubGlobal('requestAnimationFrame', (cb: FrameRequestCallback): number => {
+      rafCallbacks.push(cb)
+      return rafCallbacks.length
+    })
+    vi.stubGlobal('cancelAnimationFrame', vi.fn())
+  })
+
+  afterEach(() => {
+    act(() => {
+      root.unmount()
+    })
+    window.removeEventListener(SYNC_FIT_PANES_EVENT, onSyncFit)
+    container.remove()
+    vi.unstubAllGlobals()
+    vi.restoreAllMocks()
+  })
+
+  it('dispatches SYNC_FIT_PANES_EVENT when a split collapses to fewer leaves', () => {
+    renderLayout(SPLIT_2)
+    expect(dispatched).toBe(0)
+
+    renderLayout(LEAF)
+    flushRaf()
+    expect(dispatched).toBe(1)
+  })
+
+  it('dispatches when a nested split loses one leaf', () => {
+    renderLayout(SPLIT_3)
+    renderLayout(SPLIT_2)
+    flushRaf()
+    expect(dispatched).toBe(1)
+  })
+
+  it('does not dispatch when the leaf count increases (a split is created)', () => {
+    renderLayout(LEAF)
+    renderLayout(SPLIT_2)
+    flushRaf()
+    expect(dispatched).toBe(0)
+  })
+
+  it('does not dispatch when the leaf count is unchanged (e.g. ratio drag)', () => {
+    renderLayout(SPLIT_2)
+    renderLayout({ ...SPLIT_2, ratio: 0.3 })
+    flushRaf()
+    expect(dispatched).toBe(0)
+  })
+
+  it('does not dispatch when the worktree is not active', () => {
+    renderLayout(SPLIT_2, false)
+    renderLayout(LEAF, false)
+    flushRaf()
+    expect(dispatched).toBe(0)
+  })
+})
diff --git a/src/renderer/src/components/tab-group/use-refit-on-split-collapse.ts b/src/renderer/src/components/tab-group/use-refit-on-split-collapse.ts
new file mode 100644
index 00000000000..398ecee3c07
--- /dev/null
+++ b/src/renderer/src/components/tab-group/use-refit-on-split-collapse.ts
@@ -0,0 +1,47 @@
+import { useEffect, useRef } from 'react'
+import type { TabGroupLayoutNode } from '../../../../shared/types'
+import { SYNC_FIT_PANES_EVENT } from '@/constants/terminal'
+import { countLayoutLeaves } from './tab-group-layout-leaf-count'
+
+/**
+ * Forces a synchronous fit of all visible terminal panes when a tab-group split
+ * collapses (the layout tree loses a leaf), so the surviving group's xterm grid
+ * reflows to the new, wider container.
+ *
+ * Why: a collapsing split widens the surviving group via flexbox alone. The
+ * within-tab pane close has a deterministic rAF refit (onLayoutChanged →
+ * queueResizeAll → fitPanes); the group path otherwise relies only on a
+ * 150ms-debounced ResizeObserver that can miss the flex-only resize, leaving an
+ * idle agent's xterm pinned at the old narrow width with a blank survivor half
+ * (#6154). Double-rAF: the first frame commits React/flex styles, the second
+ * runs after layout reflow so proposeDimensions reads the new width. Only fires
+ * on a leaf-count *decrease* so splits/ratio drags (which have their own fit
+ * paths) don't trigger it, and only while the worktree is visible (the
+ * SYNC_FIT_PANES_EVENT listener and safeFit skip unmeasurable hidden panes).
+ */
+export function useRefitOnSplitCollapse(
+  layout: TabGroupLayoutNode,
+  isWorktreeActive: boolean
+): void {
+  const prevLeafCountRef = useRef(countLayoutLeaves(layout))
+  useEffect(() => {
+    const leafCount = countLayoutLeaves(layout)
+    const collapsed = leafCount < prevLeafCountRef.current
+    prevLeafCountRef.current = leafCount
+    if (!isWorktreeActive || !collapsed) {
+      return
+    }
+    const handles = { raf1: 0, raf2: 0 }
+    handles.raf1 = requestAnimationFrame(() => {
+      handles.raf2 = requestAnimationFrame(() => {
+        window.dispatchEvent(new Event(SYNC_FIT_PANES_EVENT))
+      })
+    })
+    return () => {
+      cancelAnimationFrame(handles.raf1)
+      if (handles.raf2) {
+        cancelAnimationFrame(handles.raf2)
+      }
+    }
+  }, [layout, isWorktreeActive])
+}