[Keyboard Manager] Replace text update (#46046)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request
Doing multiple text replacements in a row was unresponsive and laggy.
This PR addresses that issue.

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist
- [x] Closes: #46031 

## Validation steps
- Tested before according to the above issue, observed the bug
- Tested and refined the experience manually both locally and with the
exe from the CI build

src/modules/keyboardmanager/common/Helpers.cpp

@@ -1,5 +1,8 @@
 #include "pch.h"
 #include "Helpers.h"
+#include <atomic>
+#include <condition_variable>
+#include <queue>
 #include <sstream>
 #include <mutex>
 #include <thread>
@@ -410,69 +413,127 @@ namespace Helpers
         SendInput(ARRAYSIZE(pasteInputs), pasteInputs, sizeof(INPUT));
     }
 
-    // Serializes clipboard operations so rapid text remappings don't race.
-    static std::mutex clipboardMutex;
+    // Single background worker that processes clipboard-paste requests
+    // sequentially.  Keeping one thread avoids creating/tearing-down a
+    // thread on every keypress and naturally serializes clipboard access
+    // without a separate mutex.
+    static std::queue<std::wstring> s_clipboardQueue;
+    static std::mutex s_queueMutex;
+    static std::condition_variable s_queueCV;
+    static std::once_flag s_workerInitFlag;
+    static std::atomic<bool> s_shutdown{ false };
+    static std::thread s_workerThread;
 
-    // Function to send text via clipboard paste (Ctrl+V).
-    // Saves the previous clipboard content and restores it asynchronously.
-    // The clipboard entry is excluded from clipboard history via
-    // ExcludeClipboardContentFromMonitorProcessing (set in SetClipboardText).
-    bool SendTextViaClipboard(const std::wstring& text)
+    static void ClipboardWorkerLoop()
     {
-        // Acquire the mutex so that the entire snapshot-paste-restore cycle
-        // is atomic with respect to other text remapping calls.
-        std::unique_lock<std::mutex> lock(clipboardMutex);
-
-        // Snapshot current clipboard state
-        bool hadOriginalText = false;
-        std::wstring originalClipboardText;
-        if (OpenClipboard(nullptr))
+        while (true)
         {
-            if (IsClipboardFormatAvailable(CF_UNICODETEXT))
+            std::wstring text;
             {
-                HANDLE hData = GetClipboardData(CF_UNICODETEXT);
-                if (hData)
+                std::unique_lock<std::mutex> lock(s_queueMutex);
+                s_queueCV.wait(lock, [] { return !s_clipboardQueue.empty() || s_shutdown.load(); });
+                if (s_shutdown.load())
                 {
-                    wchar_t* pText = static_cast<wchar_t*>(GlobalLock(hData));
-                    if (pText)
+                    break;
+                }
+                text = std::move(s_clipboardQueue.front());
+                s_clipboardQueue.pop();
+            }
+
+            try
+            {
+                // Snapshot current clipboard state
+                bool hadOriginalText = false;
+                std::wstring originalClipboardText;
+                if (OpenClipboard(nullptr))
+                {
+                    if (IsClipboardFormatAvailable(CF_UNICODETEXT))
                     {
-                        originalClipboardText = pText;
-                        hadOriginalText = true;
-                        GlobalUnlock(hData);
+                        HANDLE hData = GetClipboardData(CF_UNICODETEXT);
+                        if (hData)
+                        {
+                            wchar_t* pText = static_cast<wchar_t*>(GlobalLock(hData));
+                            if (pText)
+                            {
+                                originalClipboardText = pText;
+                                hadOriginalText = true;
+                                GlobalUnlock(hData);
+                            }
+                        }
+                    }
+                    CloseClipboard();
+                }
+
+                // Place our text on the clipboard (with history exclusion)
+                if (!SetClipboardText(text))
+                {
+                    continue;
+                }
+
+                SendPasteKeystroke();
+
+                // Ctrl+V is asynchronous (SendInput queues the input), so the
+                // target app needs time to process the keystroke and read the
+                // clipboard before we restore the original content.
+                Sleep(100);
+
+                if (hadOriginalText)
+                {
+                    SetClipboardText(originalClipboardText);
+                }
+                else
+                {
+                    if (OpenClipboard(nullptr))
+                    {
+                        EmptyClipboard();
+                        CloseClipboard();
                     }
                 }
             }
-            CloseClipboard();
-        }
-
-        // Place our text on the clipboard (with history exclusion)
-        if (!SetClipboardText(text))
-        {
-            return false;
-        }
-        
-        SendPasteKeystroke();
-
-        // Restore clipboard after a delay on a background thread.
-        // Ctrl+V is asynchronous (SendInput queues the input), so the target
-        // app needs time to process the keystroke and read the clipboard.
-        // The lock is moved into the thread so the next call blocks until
-        // restoration completes.
-        std::thread([lock = std::move(lock), originalClipboardText = std::move(originalClipboardText), hadOriginalText]() {
-            Sleep(500);
-            if (hadOriginalText)
+            catch (const std::exception& ex)
             {
-                SetClipboardText(originalClipboardText);
+                OutputDebugStringA("KBM ClipboardWorker exception: ");
+                OutputDebugStringA(ex.what());
+                OutputDebugStringA("\n");
             }
-            else
+            catch (...)
             {
-                if (OpenClipboard(nullptr))
+                OutputDebugStringA("KBM ClipboardWorker unknown exception\n");
+            }
+        }
+    }
+
+    // Function to send text via clipboard paste (Ctrl+V).
+    // Saves the previous clipboard content and restores it asynchronously.
+    bool SendTextViaClipboard(const std::wstring& text)
+    {
+        // Lazily start the worker on first use.
+        std::call_once(s_workerInitFlag, [] {
+            s_workerThread = std::thread(ClipboardWorkerLoop);
+            std::atexit([] {
                 {
-                    EmptyClipboard();
-                    CloseClipboard();
+                    std::lock_guard<std::mutex> lock(s_queueMutex);
+                    s_shutdown.store(true);
                 }
+                s_queueCV.notify_one();
+                if (s_workerThread.joinable())
+                {
+                    s_workerThread.join();
+                }
+            });
+        });
+
+        // Enqueue the text and return immediately so we never block the
+        // low-level keyboard hook (WH_KEYBOARD_LL).  
+        {
+            std::lock_guard<std::mutex> lock(s_queueMutex);
+            if (!s_clipboardQueue.empty())
+            {
+                return true;
             }
-        }).detach();
+            s_clipboardQueue.push(text);
+        }
+        s_queueCV.notify_one();
 
         return true;
     }