Transfer: Fix stuck "Scanning" dialog race

The progress dialog could get stuck in "Scanning 0 files, 0 dirs" forever
when the user confirmed (or Playwright clicked) before `startScanPreview`
IPC returned. In that case, `previewId` was null when `TransferProgressDialog`
mounted. The dialog's null-recovery strategy was to adopt the ID from the
first scan event — but if those events fired between listener setup and
listener registration, they were lost, `previewId` stayed null, the status
check was skipped, and the dialog waited forever for events that never came.

- `TransferDialog.handleConfirm` now awaits the `startScan()` promise before
  calling `onConfirm`, guaranteeing `previewId` is set (or scan errored)
- Double-confirm guard added since `handleConfirm` is now async
- `TransferProgressDialog` no longer tries to adopt `previewId` from events.
  `isOurScanEvent` simplified, `effectivePreviewId` removed. The null case
  now logs an error and falls through to `startOperation`

Consistent with the dialog's own `filterEvent` pattern for write operations:
subscribe first, then dispatch the command, then match events by known ID.

Author: vdavid

apps/desktop/src/lib/file-operations/transfer/TransferDialog.svelte

@@ -114,6 +114,11 @@
     let isScanning = $state(false)
     let scanComplete = $state(false)
     let unlisteners: UnlistenFn[] = []
+    // Promise that resolves once startScanPreview IPC has returned and previewId is set.
+    // handleConfirm awaits this to guarantee previewId is non-null when passed to
+    // TransferProgressDialog — otherwise a fast confirm races with IPC and leaves the
+    // progress dialog stuck in "Scanning 0 files" forever.
+    let scanStarted: Promise<void> = Promise.resolve()
 
     // Whether the user confirmed (so we don't cancel the scan on destroy)
     let confirmed = false
@@ -332,13 +337,14 @@
 
         // Volume space is loaded by the $effect watching selectedVolumeId
 
-        // Start scanning files immediately
-        void startScan()
+        // Start scanning files immediately. Track the promise so handleConfirm can
+        // await it — this ensures previewId is set before onConfirm fires.
+        scanStarted = startScan()
 
         // Auto-confirm if MCP requested it (after a tick so the dialog is fully initialized)
         if (autoConfirm) {
             await tick()
-            handleConfirm()
+            await handleConfirm()
         }
     })
 
@@ -352,10 +358,14 @@
         cleanup()
     })
 
-    function handleConfirm() {
-        if (pathError) return
+    async function handleConfirm() {
+        if (pathError || confirmed) return
         confirmed = true
-        // Pass the previewId, conflict policy, (possibly toggled) operation type, and whether scan is still running
+        // Wait for startScanPreview IPC to return so previewId is set. Without this,
+        // a fast confirm (auto-confirm, Playwright test, rapid Enter keypress) races
+        // with the IPC and leaves the progress dialog with a null previewId that it
+        // cannot recover from once scan events have already been emitted.
+        await scanStarted
         onConfirm(editedPath, selectedVolumeId, previewId, conflictPolicy, activeOperationType, isScanning)
     }
 
@@ -370,15 +380,15 @@
 
     function handleKeydown(event: KeyboardEvent) {
         if (event.key === 'Enter') {
-            handleConfirm()
+            void handleConfirm()
         }
     }
 
     function handleInputKeydown(event: KeyboardEvent) {
         if (event.key === 'Enter') {
             event.preventDefault()
             event.stopPropagation()
-            handleConfirm()
+            void handleConfirm()
         }
     }
 </script>

apps/desktop/src/lib/file-operations/transfer/TransferProgressDialog.svelte

@@ -144,8 +144,6 @@
     let scanDirsFound = $state(0)
     let scanBytesFound = $state(0)
     let scanUnlisteners: UnlistenFn[] = []
-    /** Mutable copy of previewId — may be adopted from the first scan event if the prop was null. */
-    let effectivePreviewId: string | null = previewId
 
     // Operation state
     let operationId = $state<string | null>(null)
@@ -406,12 +404,12 @@
         const maxConflictsToShow = getSetting('fileOperations.maxConflictsToShow')
 
         if (operationType === 'trash') {
-            return trashFiles(sourcePaths, itemSizes, { progressIntervalMs, previewId: effectivePreviewId })
+            return trashFiles(sourcePaths, itemSizes, { progressIntervalMs, previewId })
         }
         if (operationType === 'delete') {
             return deleteFiles(
                 sourcePaths,
-                { progressIntervalMs, sortColumn, sortOrder, previewId: effectivePreviewId },
+                { progressIntervalMs, sortColumn, sortOrder, previewId },
                 sourceVolumeId,
             )
         }
@@ -427,7 +425,7 @@
                         conflictResolution,
                         progressIntervalMs,
                         maxConflictsToShow,
-                        previewId: effectivePreviewId,
+                        previewId,
                     },
                 )
             }
@@ -438,7 +436,7 @@
                 maxConflictsToShow,
                 sortColumn,
                 sortOrder,
-                previewId: effectivePreviewId,
+                previewId,
             })
         }
         // Copy: always use copyBetweenVolumes — the backend handles local-to-local optimization
@@ -451,7 +449,7 @@
                 conflictResolution,
                 progressIntervalMs,
                 maxConflictsToShow,
-                previewId: effectivePreviewId,
+                previewId,
             },
         )
     }
@@ -517,9 +515,9 @@
 
     async function handleCancel(rollback: boolean) {
         // If still waiting for scan preview, cancel the scan and close
-        if (waitingForScan && effectivePreviewId) {
-            log.info('Cancelling scan preview during wait: previewId={previewId}', { previewId: effectivePreviewId })
-            void cancelScanPreview(effectivePreviewId)
+        if (waitingForScan && previewId) {
+            log.info('Cancelling scan preview during wait: previewId={previewId}', { previewId })
+            void cancelScanPreview(previewId)
             waitingForScan = false
             cleanupScanListeners()
             onCancelled(0)
@@ -622,21 +620,26 @@
         scanUnlisteners = []
     }
 
-    /** Returns true if the event belongs to our scan preview, adopting the ID if we don't have one yet. */
+    /** Returns true if the event belongs to our scan preview. */
     function isOurScanEvent(eventPreviewId: string): boolean {
-        if (!effectivePreviewId) effectivePreviewId = eventPreviewId
-        return eventPreviewId === effectivePreviewId
+        return eventPreviewId === previewId
     }
 
     /**
      * Waits for the scan preview to complete, then starts the write operation.
      *
      * Subscribes to scan events BEFORE checking status to avoid the race where
      * the scan completes between the status check and listener registration.
-     * If previewId is null (TransferDialog confirmed before startScanPreview IPC
-     * returned), adopts the first event's previewId.
+     *
+     * Precondition: previewId must be non-null (guaranteed by TransferDialog,
+     * which awaits startScanPreview IPC before calling onConfirm).
      */
     async function waitForScanThenStart() {
+        if (!previewId) {
+            log.error('waitForScanThenStart called with null previewId — TransferDialog invariant violated')
+            void startOperation()
+            return
+        }
         // Subscribe to events FIRST to avoid missing fast completions.
         // Same pattern as TransferDialog.startScan().
         scanUnlisteners.push(
@@ -690,21 +693,17 @@
         )
 
         // NOW check if already complete (covers race where scan finished during subscription setup)
-        if (effectivePreviewId) {
-            const alreadyComplete = await checkScanPreviewStatus(effectivePreviewId)
-            if (alreadyComplete) {
-                log.info('Scan preview already complete for previewId={previewId}, starting operation immediately', {
-                    previewId: effectivePreviewId,
-                })
-                cleanupScanListeners()
-                void startOperation()
-                return
-            }
+        const alreadyComplete = await checkScanPreviewStatus(previewId)
+        if (alreadyComplete) {
+            log.info('Scan preview already complete for previewId={previewId}, starting operation immediately', {
+                previewId,
+            })
+            cleanupScanListeners()
+            void startOperation()
+            return
         }
 
-        log.info('Scan preview still running for previewId={previewId}, subscribing to events', {
-            previewId: effectivePreviewId,
-        })
+        log.info('Scan preview still running for previewId={previewId}, subscribing to events', { previewId })
         waitingForScan = true
     }
 
@@ -719,8 +718,8 @@
     onDestroy(() => {
         destroyed = true
         // Cancel scan preview if still waiting for it
-        if (waitingForScan && effectivePreviewId) {
-            void cancelScanPreview(effectivePreviewId)
+        if (waitingForScan && previewId) {
+            void cancelScanPreview(previewId)
         }
         cleanupScanListeners()
         if (operationId && !operationSettled) {