fix(state): 拖动窗格内最后一个 tab 到自身边缘会直接丢失该 tab

src/client/state.ts

@@ -270,23 +270,35 @@ export function moveTabToEdge(
   const source = leafWithTab(node, tabId)
   if (source === undefined) return state
   const tab = source.tabs.find(candidate => candidate.id === tabId)!
-  let emptied = false
-  let splits = mapLeaf(node, source.id, (leaf) => {
-    leaf.tabs = leaf.tabs.filter(candidate => candidate.id !== tabId)
-    if (leaf.active === tabId) leaf.active = leaf.tabs[leaf.tabs.length - 1]?.id ?? null
-    if (leaf.tabs.length === 0) emptied = true
-  })
-  if (emptied) splits = removeLeafAt(splits, source.id)
   if (zone === 'center') {
+    let emptied = false
+    let splits = mapLeaf(node, source.id, (leaf) => {
+      leaf.tabs = leaf.tabs.filter(candidate => candidate.id !== tabId)
+      if (leaf.active === tabId) leaf.active = leaf.tabs[leaf.tabs.length - 1]?.id ?? null
+      if (leaf.tabs.length === 0) emptied = true
+    })
+    if (emptied) splits = removeLeafAt(splits, source.id)
     splits = mapLeaf(splits, toPane, (leaf) => {
       leaf.tabs = [...leaf.tabs, tab]
       leaf.active = tab.id
     })
     return { ...state, bottomSplits: splits, activePane: toPane }
   }
+  // Edge zones split the target pane first, then lift the tab out of its
+  // source pane. Doing it the other way around breaks a self-drop: removing
+  // the tab empties the source leaf and removeLeafAt deletes it from the
+  // tree, so insertLeafAt targets an id that no longer exists and the
+  // dragged tab is silently discarded.
   const dir = zone === 'left' || zone === 'right' ? 'row' : 'col'
-  const result = insertLeafAt(splits, toPane, dir, tab, zone === 'left' || zone === 'up')
-  return { ...state, bottomSplits: result.node, activePane: result.leafId }
+  const inserted = insertLeafAt(node, toPane, dir, tab, zone === 'left' || zone === 'up')
+  let emptied = false
+  let splits = mapLeaf(inserted.node, source.id, (leaf) => {
+    leaf.tabs = leaf.tabs.filter(candidate => candidate.id !== tabId)
+    if (leaf.active === tabId) leaf.active = leaf.tabs[leaf.tabs.length - 1]?.id ?? null
+    if (leaf.tabs.length === 0) emptied = true
+  })
+  if (emptied) splits = removeLeafAt(splits, source.id)
+  return { ...state, bottomSplits: splits, activePane: inserted.leafId }
 }
 
 /**

tests/state.spec.ts

@@ -247,6 +247,40 @@ describe('sidebar state', () => {
     expect(after.tabs).toHaveLength(2)
   })
 
+  it('dragging a pane\'s only tab onto its own edge splits in place instead of losing the tab', () => {
+    let s = state()
+    s = openTabInBottomPane(s, { id: 'git', type: 'git', title: 'Git' })
+    s = splitPane(s, 'col')
+    const split = s.bottomSplits as Extract<SplitNode, { kind: 'split' }>
+    const paneA = split.children[0] as { id: string; tabs: { id: string }[] }
+    const paneB = split.children[1] as { id: string }
+    const tabId = paneA.tabs[0]!.id
+    s = moveTab(s, paneA.id, tabId, paneB.id)
+    // paneB now holds the single tab; dropping it onto paneB's own edge
+    // used to empty paneB, delete its leaf, and discard the tab entirely.
+    s = moveTabToEdge(s, paneB.id, tabId, paneB.id, 'left')
+    const leaves = allLeaves(s.bottomSplits)
+    expect(leaves).toHaveLength(1)
+    expect(leaves[0]!.tabs.map(t => t.id)).toEqual([tabId])
+    expect(s.activePane).toBe(leaves[0]!.id)
+  })
+
+  it('dragging one of several tabs onto its own pane edge splits the pane with the dragged tab', () => {
+    let s = state()
+    s = openTabInBottomPane(s, { id: 'git', type: 'git', title: 'Git' })
+    s = openTabInBottomPane(s, { id: 't2', type: 'terminal', title: 'T2' })
+    const leaf = s.bottomSplits as { id: string; tabs: { id: string }[] }
+    const first = leaf.tabs[0]!.id
+    s = moveTabToEdge(s, leaf.id, first, leaf.id, 'right')
+    const leaves = allLeaves(s.bottomSplits)
+    expect(leaves).toHaveLength(2)
+    const dragged = leaves.find(candidate => candidate.tabs.some(t => t.id === first))
+    const kept = leaves.find(candidate => candidate !== dragged)
+    expect(dragged!.tabs.map(t => t.id)).toEqual([first])
+    expect(kept!.tabs.map(t => t.id)).toEqual([leaf.tabs[1]!.id])
+    expect(s.activePane).toBe(dragged!.id)
+  })
+
   it('closing the last tab removes the pane (promotes the sibling)', () => {
     let s = state()
     s = openTabInBottomPane(s, { id: 'git', type: 'git', title: 'Git' })