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fix(mobile): blank-terminal-but-input-works race

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Three independent bugs that combined to cause the rare blank-terminal
state where xterm shows nothing but server input still works:

1. Scrollback handler set initializedHandlesRef even when getTerminalRef
   returned null (WebView not mounted). The init() message went nowhere
   but the flag stayed true, so any subsequent scrollback for the handle
   was silently dropped — xterm never got a buffer. Now only mark
   initialized when init() actually reaches a live WebView ref.

2. Stale-event filter dropped 'scrollback' events with seq < last when
   the regress was <= 20. After a WS reconnect or resubscribe the server
   may emit scrollback at a lower seq than what we saw pre-reconnect,
   leaving the user with a blank terminal. Scrollback is the response
   to a fresh subscribe — it should always reset the high-water mark.
   The filter now only applies to 'resized' events.

3. Added defensive logging in the 'data' handler so the next time blank
   happens we can see whether sends are being silently dropped because
   getTerminalRef returned null.

Co-authored-by: Orca <help@stably.ai>
This commit is contained in:
Jinwoo-H
2026-05-06 20:12:45 -04:00
co-authored by Orca
parent ad7ec5775e
commit d51628c889
1 changed files with 55 additions and 9 deletions
Download Patch File Download Diff File Expand all files Collapse all files
mobile/app/h/[hostId]/session/[worktreeId].tsx
+55 -9
View File
@@ -260,13 +260,16 @@ export default function SessionScreen() {
if (subscribeSeqRef.current.get(handle) !== seq) return
const data = result as Record<string, unknown>
// Why: stale-event filter. Server-side state machine bumps a
// monotonic seq on every applyLayout. Drop scrollback/resized
// events whose seq is strictly older than what we've already
// observed for this handle — they're late-arriving from a
// superseded layout. >20-event regress = treat as server
// restart (fresh state machine), reset the high-water mark.
// monotonic seq on every applyLayout. Drop `resized` events
// whose seq is strictly older than what we've already observed
// for this handle — they're late-arriving from a superseded
// layout. `scrollback` is the response to a fresh subscribe,
// so it always resets the high-water mark regardless of seq
// (post-WS-reconnect or post-resubscribe the server may emit
// scrollback at a seq lower than what we'd seen pre-reconnect;
// dropping it would leave the user with a blank terminal).
const eventSeq = typeof data.seq === 'number' ? data.seq : null
if (eventSeq != null && (data.type === 'scrollback' || data.type === 'resized')) {
if (eventSeq != null && data.type === 'resized') {
const last = layoutSeqRef.current.get(handle)
if (last != null && eventSeq < last && last - eventSeq <= 20) {
console.log('[fit][session] DROP-stale-seq', {
@@ -281,6 +284,8 @@ export default function SessionScreen() {
return
}
layoutSeqRef.current.set(handle, eventSeq)
} else if (eventSeq != null && data.type === 'scrollback') {
layoutSeqRef.current.set(handle, eventSeq)
}
if (data.type === 'scrollback' || data.type === 'resized') {
console.log('[fit][session] event', {
@@ -294,7 +299,14 @@ export default function SessionScreen() {
})
}
if (data.type === 'scrollback') {
if (initializedHandlesRef.current.has(handle)) return
if (initializedHandlesRef.current.has(handle)) {
console.log('[fit][session] scrollback IGNORED (already initialized)', {
handle: handle.slice(-8),
cols: data.cols,
rows: data.rows
})
return
}
const cols = (data.cols as number) || 80
const rows = (data.rows as number) || 24
const scrollbackCols = cols
@@ -303,7 +315,21 @@ export default function SessionScreen() {
typeof data.serialized === 'string' && data.serialized.length > 0
? data.serialized
: ''
getTerminalRef(handle)?.init(cols, rows, initialData)
const ref = getTerminalRef(handle)
// Why: previously we set `initializedHandlesRef` even when the
// WebView wasn't mounted yet (ref=null). The init message went
// nowhere, but the flag stayed true, so any subsequent scrollback
// for THIS handle was silently dropped → blank terminal. Only
// mark initialized if init() actually reached the WebView.
if (!ref) {
console.log('[fit][session] scrollback DROPPED — no terminal ref', {
handle: handle.slice(-8),
cols,
rows
})
return
}
ref.init(cols, rows, initialData)
initializedHandlesRef.current.add(handle)
if (data.displayMode) {
setTerminalModes((prev) =>
@@ -374,7 +400,27 @@ export default function SessionScreen() {
})()
}
} else if (data.type === 'data') {
getTerminalRef(handle)?.write(data.chunk as string)
// Why: log when data arrives but the WebView ref is missing
// — this is the most likely cause of "blank but input works":
// server stream is alive, sends flow, but writes are dropped
// because the WebView ref disappeared (unmount mid-flight) or
// the scrollback never landed (so xterm has no buffer).
const dataRef = getTerminalRef(handle)
if (!dataRef) {
console.log('[fit][session] data DROPPED — no terminal ref', {
handle: handle.slice(-8),
chunkLen: typeof data.chunk === 'string' ? data.chunk.length : 0,
initialized: initializedHandlesRef.current.has(handle)
})
return
}
if (!initializedHandlesRef.current.has(handle)) {
console.log('[fit][session] data RECEIVED before scrollback', {
handle: handle.slice(-8),
chunkLen: typeof data.chunk === 'string' ? data.chunk.length : 0
})
}
dataRef.write(data.chunk as string)
} else if (data.type === 'resized') {
// Why: inline resize event — the server changed the PTY dimensions
// (mode toggle or desktop restore). Reinitialize xterm at the new