fix(mailbox): show spinner during initial folder fetch instead of empty state (#167)

selectFolder() kicked off its background syncFolder() fetch fire-and-forget
with no loading flag, so opening a per-account folder with no cached
messages yet flashed "No messages to display" for the whole IMAP fetch
instead of a spinner. Adds isSyncingFolder, a StateFlow set for the
duration of that sync (mirroring isRefreshing) and cleared via try/finally
regardless of outcome. A private latestFolderSelection token guards the
clear so a stale sync from a folder no longer selected can't hide the
spinner for whichever folder is actually selected now.

MailboxScreen's non-paged empty branch now holds NoMessagesState back
while isSyncingFolder is true, showing a CircularProgressIndicator
instead — mirroring how the unified-inbox paged branch already gates on
loadState.refresh.

Closes #149

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

app/src/main/kotlin/org/libremail/ui/mailbox/MailboxScreen.kt

@@ -120,6 +120,7 @@ fun MailboxScreen(
     val searchActive by viewModel.searchActive.collectAsStateWithLifecycle()
     val searchQuery by viewModel.searchQuery.collectAsStateWithLifecycle()
     val isRefreshing by viewModel.isRefreshing.collectAsStateWithLifecycle()
+    val isSyncingFolder by viewModel.isSyncingFolder.collectAsStateWithLifecycle()
     val error by viewModel.error.collectAsStateWithLifecycle()
     val selectedIds by viewModel.selectedIds.collectAsStateWithLifecycle()
     val pendingConfirm by viewModel.pendingConfirm.collectAsStateWithLifecycle()
@@ -330,8 +331,16 @@ fun MailboxScreen(
                                     }
                                 } else if (messages.isEmpty()) {
                                     item {
+                                        // Hold the empty state back while the folder's initial background
+                                        // sync is still in flight, so opening an uncached folder doesn't
+                                        // flash "No messages to display" before the fetch has a chance to
+                                        // populate anything (issue #149).
                                         if (searchActive && searchQuery.isNotBlank()) {
                                             NoResultsState(Modifier.fillParentMaxSize())
+                                        } else if (isSyncingFolder) {
+                                            Box(Modifier.fillParentMaxSize(), contentAlignment = Alignment.Center) {
+                                                CircularProgressIndicator()
+                                            }
                                         } else {
                                             NoMessagesState(Modifier.fillParentMaxSize())
                                         }

app/src/main/kotlin/org/libremail/ui/mailbox/MailboxViewModel.kt

@@ -186,6 +186,22 @@ class MailboxViewModel @Inject constructor(
     private val _isRefreshing = MutableStateFlow(false)
     val isRefreshing: StateFlow<Boolean> = _isRefreshing.asStateFlow()
 
+    /**
+     * True while [selectFolder]'s background sync for the currently selected folder is in flight,
+     * so the screen can hold the empty state back until the initial fetch either populates the
+     * cache or confirms the folder really is empty (issue #149).
+     */
+    private val _isSyncingFolder = MutableStateFlow(false)
+    val isSyncingFolder: StateFlow<Boolean> = _isSyncingFolder.asStateFlow()
+
+    /**
+     * (accountId, folder) of the most recent [selectFolder] call. Used only so a completing sync
+     * can tell whether it's stale — superseded by a newer folder selection — before clearing
+     * [_isSyncingFolder], so rapid folder switching can't let a stale sync hide the spinner for
+     * whichever folder is actually selected now.
+     */
+    private var latestFolderSelection: Pair<String, String>? = null
+
     private val _error = MutableStateFlow<String?>(null)
     val error: StateFlow<String?> = _error.asStateFlow()
 
@@ -365,7 +381,18 @@ class MailboxViewModel @Inject constructor(
         _selectedAccountId.value = accountId
         explicitDrawerAccountId.value = accountId
         _selectedFolder.value = folderFullName
-        viewModelScope.launch { mailSyncer.syncFolder(accountId, folderFullName) }
+        val key = accountId to folderFullName
+        latestFolderSelection = key
+        _isSyncingFolder.value = true
+        viewModelScope.launch {
+            try {
+                mailSyncer.syncFolder(accountId, folderFullName)
+            } finally {
+                // Only clear if this is still the most recent selection — a stale sync superseded
+                // by another selectFolder() must not clobber the newer one's in-flight spinner.
+                if (latestFolderSelection == key) _isSyncingFolder.value = false
+            }
+        }
     }
 
     /** Returns to the unified inbox across all accounts. */