fix(desktop): prefer the live backlog over the last snapshot fence when appending a catch-up delta

syncAgentTerminalSnapshot checked the stale lastSnapshots prefix-delta before
the authoritative backlog (body) prefix-delta. Live agent.terminal.output
chunks advance the backlog but never lastSnapshots, so once any chunk has
streamed since the last snapshot, body is strictly longer than previous and
both are prefixes of a catch-up process.list snapshot. The fence-first branch
then emitted output.slice(previous.length), re-appending bytes the chunks had
already written and duplicating them on screen and in the replay backlog.

body is the authoritative on-screen state and is always >= previous, so its
delta is the correct, smaller one; check it first. Regression case added to
agent-terminal-stream.bound.test.ts (red without the swap).

Co-authored-by: briandevans <252620095+briandevans@users.noreply.github.com>

apps/desktop/src/app/right-sidebar/terminal/agent-terminal-stream.bound.test.ts

@@ -133,6 +133,26 @@ describe('agent terminal retention is bounded', () => {
     expect(replay('reset-11')).toContain(tail.slice(0, 1000))
   })
 
+  it('a catch-up snapshot does not re-print bytes already streamed by live chunks', () => {
+    // Live `agent.terminal.output` chunks advance the backlog but never the
+    // lastSnapshots fence. A catch-up snapshot that contains everything shown so
+    // far matches BOTH prefix checks, and the fence-first order emitted
+    // output.slice(previous.length) — re-appending bytes the chunks had already
+    // written, so the screen showed `AAABBBAAABBBCCC` for `AAABBB` + `CCC`.
+    const proc = 'dup-proc'
+    let screen = ''
+
+    const stop = registerAgentTerminalWriter(proc, chunk => {
+      screen = chunk.startsWith('\x1bc') ? chunk.slice(2) : screen + chunk
+    })
+
+    writeAgentTerminalChunk(proc, 'AAA')
+    writeAgentTerminalChunk(proc, 'BBB')
+    syncAgentTerminalSnapshot(proc, 'AAABBBCCC')
+    stop()
+    expect(screen).toBe('AAABBBCCC')
+  })
+
   it('an evicted process does not diff against a tail it no longer has', () => {
     // `lastSnapshots` is the delta fence for `backlog`. If eviction dropped the
     // backlog but KEPT the fence, the next snapshot takes the `startsWith(previous)`

apps/desktop/src/app/right-sidebar/terminal/agent-terminal-stream.ts

@@ -137,18 +137,22 @@ export function syncAgentTerminalSnapshot(procId: string, output: string): void
     return
   }
 
-  // The fence is updated BEFORE the write so the write's eviction pass counts it:
-  // after, it would re-create a fence for a process that pass just forgot.
-  if (output.startsWith(previous)) {
+  // The live backlog (body) is the authoritative on-screen state and is always at
+  // least as long as the last snapshot fence, so check its prefix-delta FIRST: the
+  // fence-first order would re-append bytes already streamed by live chunks and
+  // print them twice. The fence is updated BEFORE the write so the write's eviction
+  // pass counts it: after, it would re-create a fence for a process that pass just
+  // forgot.
+  if (output.startsWith(body)) {
     lastSnapshots.set(procId, output)
-    writeAgentTerminalChunk(procId, output.slice(previous.length))
+    writeAgentTerminalChunk(procId, output.slice(body.length))
 
     return
   }
 
-  if (output.startsWith(body)) {
+  if (output.startsWith(previous)) {
     lastSnapshots.set(procId, output)
-    writeAgentTerminalChunk(procId, output.slice(body.length))
+    writeAgentTerminalChunk(procId, output.slice(previous.length))
 
     return
   }