diff --git a/src/ngui.zig b/src/ngui.zig
index 1b8e2de..6f8936d 100644
--- a/src/ngui.zig
+++ b/src/ngui.zig
@@ -220,7 +220,6 @@ fn uiThreadLoop() void {
                 comm.write(gpa, stdout, comm.Message.standby) catch |err| {
                     logger.err("comm.write standby: {any}", .{err});
                 };
-                // MUTEX???????????????????????????????????
                 screen.sleep(&ui_mutex, &wakeup); // blocking
 
                 // wake up due to touch screen activity or wakeup event is set
diff --git a/src/ui/screen.zig b/src/ui/screen.zig
index 7359be5..6f73f9d 100644
--- a/src/ui/screen.zig
+++ b/src/ui/screen.zig
@@ -13,18 +13,21 @@ const logger = std.log.scoped(.screen);
 /// a touch screen activity or wake event is triggered.
 /// sleep removes all input devices at enter and reinstates them at exit so that
 /// a touch event triggers no accidental action.
-pub fn sleep(mu: *std.Thread.Mutex, wake: *const Thread.ResetEvent) void {
+///
+/// the UI mutex is held while calling LVGL UI functions, and released during
+/// idling or waiting for wake event.
+/// although sleep is safe for concurrent use, the input drivers init/deinit
+/// implementation used on entry and exit might not be.
+pub fn sleep(ui: *std.Thread.Mutex, wake: *const Thread.ResetEvent) void {
+    ui.lock();
     drv.deinitInput();
-
-    mu.lock();
     widget.topdrop(.show);
-    mu.unlock();
-
+    ui.unlock();
     defer {
+        ui.lock();
+        defer ui.unlock();
         drv.initInput() catch |err| logger.err("drv.initInput: {any}", .{err});
-        mu.lock();
         widget.topdrop(.remove);
-        mu.unlock();
     }
 
     const watcher = drv.InputWatcher() catch |err| {