Let a log host say it has no verbose stream

The logs screen has always offered two streams to unfold between, on the assumption that a host can
always read the whole device log as well as the routed subset. A host that has lost the privileged
backend it reads the device through is reduced to its own process's log, where the two streams would
be the same lines under two names, and the toggle becomes a control that changes nothing.

LogSource gains hasVerboseStream, defaulting to true so every existing host keeps both. The screen
reads it live rather than through the view model, since the backend can come and go while the screen
is open: the toggle disappears with the stream, and a reader left standing on the verbose pane is
moved back to the modules one instead of staring at a stream the host no longer serves.

Author: JingMatrix

manager-ui/src/main/kotlin/org/matrix/vector/ui/logs/LogSource.kt

@@ -42,6 +42,17 @@ interface LogSource {
      */
     suspend fun open(verbose: Boolean, part: String?): Result<LogContent?>
 
+    /**
+     * Whether the host actually has a second, verbose stream to unfold into.
+     *
+     * A host can be reduced to a single log — one that reads only its own process because the
+     * privileged backend it normally reads the device through is unavailable — and then the two
+     * streams would be the same lines under two names. Saying so drops the unfold control instead of
+     * offering a choice that changes nothing.
+     */
+    val hasVerboseStream: Boolean
+        get() = true
+
     // --- Verbose-logging preference (distinct from which stream is on screen) ------------------
 
     /** Whether the host has a persistent "write verbose lines at all" preference to toggle. */

manager-ui/src/main/kotlin/org/matrix/vector/ui/logs/LogsScreen.kt

@@ -151,6 +151,14 @@ fun LogsScreen(
     // search boxes, two filter states and two scroll positions for what is one question — "what
     // does the log say" — whose answer often has to be looked for in both streams.
     var currentTab by rememberSaveable { mutableStateOf(LogTab.MODULES) }
+    // Read from the source rather than snapshotted in the view model: a host can gain or lose its
+    // verbose stream while the screen is open (a backend granted, a service lost), and the control
+    // has to follow. Losing it mid-read also has to move the reader back, or the pane would stay on
+    // a stream the host no longer serves.
+    val hasVerboseStream = source.hasVerboseStream
+    LaunchedEffect(hasVerboseStream) {
+        if (!hasVerboseStream && currentTab == LogTab.VERBOSE) currentTab = LogTab.MODULES
+    }
     val currentState by viewModel.state(currentTab).collectAsStateWithLifecycle()
     val wordWrap by viewModel.wordWrap.collectAsStateWithLifecycle()
     val saveState by viewModel.saveState.collectAsStateWithLifecycle()
@@ -223,6 +231,7 @@ fun LogsScreen(
                         state = currentState,
                         viewModel = viewModel,
                         onSelectTab = { currentTab = it },
+                        showSourceToggle = hasVerboseStream,
                     )
                 },
                 actions = {
@@ -560,14 +569,15 @@ private fun LogSearch(
     state: LogPaneState,
     viewModel: LogsViewModel,
     onSelectTab: (LogTab) -> Unit,
+    showSourceToggle: Boolean,
 ) {
     var filterOpen by remember { mutableStateOf(false) }
     SearchField(
         query = state.query.text,
         onQueryChange = { viewModel.setQuery(tab, it) },
         placeholder = stringResource(R.string.logs_search_hint),
         trailing = {
-            LogSourceToggle(tab = tab, onSelect = onSelectTab)
+            if (showSourceToggle) LogSourceToggle(tab = tab, onSelect = onSelectTab)
             IconButton(
                 onClick = {
                     filterOpen = true