perf(sidebar): stop building host projections the row model throws away

getMixedHostContextLabels built a label map for every visible worktree and then
returned undefined unless two distinct hosts existed — so a single-host install,
the common case, paid the whole build on every buildRows rebuild. Decide first,
then build only when it is mixed.

getHostWorktreeCounts repeated getHostWorktreeIds' dedupe walk to compute a
number that is exactly the id list's length, and both loops built the identity
string twice per row. Derive the counts and hoist the identity.

src/renderer/src/components/sidebar/worktree-list/grouping/host-labels.test.ts

@@ -0,0 +1,62 @@
+import { describe, expect, it } from 'vitest'
+import type { ExecutionHostId } from '../../../../../../shared/execution-host'
+import type { Repo } from '../../../../../../shared/repo-types'
+import type { Worktree } from '../../../../../../shared/worktree-types'
+import { getHostWorktreeCounts, getHostWorktreeIds } from './host-labels'
+
+const LOCAL = 'local' as ExecutionHostId
+
+function makeRepos(hostByRepo: Record<string, ExecutionHostId | undefined>): Map<string, Repo> {
+  return new Map(
+    Object.entries(hostByRepo).map(([id, executionHostId]) => [
+      id,
+      { id, path: `/${id}`, ...(executionHostId ? { executionHostId } : {}) } as Repo
+    ])
+  )
+}
+
+function makeWorktrees(rows: { id: string; repoId: string }[]): Worktree[] {
+  return rows.map((row) => row as Worktree)
+}
+
+describe('host worktree counts and ids', () => {
+  it('reports a count equal to the length of each host id list', () => {
+    const repoMap = makeRepos({
+      'repo-local': undefined,
+      'repo-remote': 'ssh:other' as ExecutionHostId
+    })
+    const worktrees = makeWorktrees([
+      { id: 'a', repoId: 'repo-local' },
+      { id: 'b', repoId: 'repo-local' },
+      { id: 'c', repoId: 'repo-remote' }
+    ])
+
+    const counts = getHostWorktreeCounts(worktrees, repoMap, LOCAL)
+    const ids = getHostWorktreeIds(worktrees, repoMap, LOCAL)
+
+    expect(ids).toBeDefined()
+    expect(counts).toBeDefined()
+    for (const [hostId, hostIds] of ids ?? []) {
+      expect(counts?.get(hostId), `count for ${hostId}`).toBe(hostIds.length)
+    }
+    expect([...(counts?.keys() ?? [])].sort()).toEqual([...(ids?.keys() ?? [])].sort())
+  })
+
+  it('counts a repeated host identity once', () => {
+    const repoMap = makeRepos({ 'repo-local': undefined })
+    const worktrees = makeWorktrees([
+      { id: 'a', repoId: 'repo-local' },
+      { id: 'a', repoId: 'repo-local' },
+      { id: 'b', repoId: 'repo-local' }
+    ])
+
+    expect(getHostWorktreeCounts(worktrees, repoMap, LOCAL)?.get(LOCAL)).toBe(2)
+    expect(getHostWorktreeIds(worktrees, repoMap, LOCAL)?.get(LOCAL)).toEqual(['a', 'b'])
+  })
+
+  it('returns undefined for an empty lane', () => {
+    const repoMap = makeRepos({})
+    expect(getHostWorktreeCounts([], repoMap, LOCAL)).toBeUndefined()
+    expect(getHostWorktreeIds([], repoMap, LOCAL)).toBeUndefined()
+  })
+})

src/renderer/src/components/sidebar/worktree-list/grouping/host-labels.ts

@@ -153,17 +153,17 @@ export function getHostWorktreeCounts(
   if (worktrees.length === 0) {
     return undefined
   }
-  const counts = new Map<ExecutionHostId, number>()
+  // Derived from the id map rather than repeating its dedupe walk: every caller asks for both,
+  // and a host's count is exactly the length of its id list by construction.
   // Dedup by host, not by bare id: the same id on two hosts is two workspaces
   // and has to be counted under each of them (STA-4343).
-  const seenIdentities = new Set<string>()
-  for (const worktree of worktrees) {
-    if (seenIdentities.has(getWorktreeHostIdentity(worktree))) {
-      continue
-    }
-    seenIdentities.add(getWorktreeHostIdentity(worktree))
-    const hostId = getWorktreeExecutionHostId(worktree, repoMap.get(worktree.repoId), defaultHostId)
-    counts.set(hostId, (counts.get(hostId) ?? 0) + 1)
+  const idsByHost = getHostWorktreeIds(worktrees, repoMap, defaultHostId)
+  if (!idsByHost) {
+    return undefined
+  }
+  const counts = new Map<ExecutionHostId, number>()
+  for (const [hostId, ids] of idsByHost) {
+    counts.set(hostId, ids.length)
   }
   return counts
 }
@@ -179,10 +179,12 @@ export function getHostWorktreeIds(
   const idsByHost = new Map<ExecutionHostId, string[]>()
   const seenIdentities = new Set<string>()
   for (const worktree of worktrees) {
-    if (seenIdentities.has(getWorktreeHostIdentity(worktree))) {
+    // Hoisted: the identity string was built twice per row, once to test and once to record.
+    const identity = getWorktreeHostIdentity(worktree)
+    if (seenIdentities.has(identity)) {
       continue
     }
-    seenIdentities.add(getWorktreeHostIdentity(worktree))
+    seenIdentities.add(identity)
     const hostId = getWorktreeExecutionHostId(worktree, repoMap.get(worktree.repoId), defaultHostId)
     const ids = idsByHost.get(hostId) ?? []
     ids.push(worktree.id)

src/shared/worktree/host-context-labels.test.ts

@@ -0,0 +1,53 @@
+import { describe, expect, it } from 'vitest'
+import type { ExecutionHostId } from '../execution-host'
+import { getMixedHostContextLabels } from './host-context-labels'
+
+type Item = { id: string; hostId: ExecutionHostId }
+
+function argsFor(items: Item[], counters: { hostIdReads: number; identityReads: number }) {
+  return {
+    getHostId: (item: Item): ExecutionHostId => {
+      counters.hostIdReads += 1
+      return item.hostId
+    },
+    getIdentity: (item: Item): string => {
+      counters.identityReads += 1
+      return item.id
+    }
+  }
+}
+
+describe('getMixedHostContextLabels', () => {
+  it('returns undefined without building any labels when every item shares one host', () => {
+    const items: Item[] = Array.from({ length: 50 }, (_, index) => ({
+      id: `wt-${index}`,
+      hostId: 'local' as ExecutionHostId
+    }))
+    const counters = { hostIdReads: 0, identityReads: 0 }
+
+    expect(getMixedHostContextLabels(items, argsFor(items, counters))).toBeUndefined()
+    // The label map was built for all 50 and thrown away before; now nothing is built.
+    expect(counters.identityReads).toBe(0)
+  })
+
+  it('labels every item once a second host appears, wherever it appears', () => {
+    for (const lateIndex of [1, 25, 49]) {
+      const items: Item[] = Array.from({ length: 50 }, (_, index) => ({
+        id: `wt-${index}`,
+        hostId: (index === lateIndex ? 'ssh:other' : 'local') as ExecutionHostId
+      }))
+      const counters = { hostIdReads: 0, identityReads: 0 }
+
+      const labels = getMixedHostContextLabels(items, argsFor(items, counters))
+
+      expect(labels, `second host at index ${lateIndex}`).toBeDefined()
+      expect(labels?.size).toBe(50)
+      expect(labels?.has(`wt-${lateIndex}`)).toBe(true)
+    }
+  })
+
+  it('returns undefined for an empty list', () => {
+    const counters = { hostIdReads: 0, identityReads: 0 }
+    expect(getMixedHostContextLabels([], argsFor([], counters))).toBeUndefined()
+  })
+})

src/shared/worktree/host-context-labels.ts

@@ -83,12 +83,30 @@ export function getMixedHostContextLabels<T>(
     sources?: HostContextLabelSources
   }
 ): Map<string, string> | undefined {
-  const labelsByIdentity = new Map<string, string>()
-  const hostIds = new Set<ExecutionHostId>()
+  // Why two passes: on a single-host install the map is built for every item and then discarded.
+  // Deciding first costs one getHostId per item and skips the label work entirely.
+  let firstHostId: ExecutionHostId | undefined
+  let isMixed = false
   for (const item of items) {
     const hostId = args.getHostId(item)
-    hostIds.add(hostId)
-    labelsByIdentity.set(args.getIdentity(item), getHostContextLabel(hostId, args.sources))
+    if (firstHostId === undefined) {
+      firstHostId = hostId
+      continue
+    }
+    if (hostId !== firstHostId) {
+      isMixed = true
+      break
+    }
   }
-  return hostIds.size > 1 ? labelsByIdentity : undefined
+  if (!isMixed) {
+    return undefined
+  }
+  const labelsByIdentity = new Map<string, string>()
+  for (const item of items) {
+    labelsByIdentity.set(
+      args.getIdentity(item),
+      getHostContextLabel(args.getHostId(item), args.sources)
+    )
+  }
+  return labelsByIdentity
 }