ImguiSystem: reset keydown state properly once window closes

In commit 1fd4c25cd0188f96c7d1fe7b0a8b072763eed032, an attempt was made to fix this by calling the message handler with WM_NULL, but WM_KILLFOCUS should've been passed instead, as Dear ImGui clears the keydown state with this message. WM_NULL does nothing.

src/engine/sys_mainwind.cpp

@@ -83,16 +83,14 @@ LRESULT CGame::ImguiWindowProc(HWND hWnd, UINT& uMsg, WPARAM wParam, LPARAM lPar
 	}//////////////////////////////////////////////////////////////////////////////
 	else
 	{
-		if (g_bBlockInput)
+		if (g_bBlockInput.exchange(false))
 		{
-			// Dry run with null msg to clear the event queue, we have to do this as
-			// the menu's can be closed while still holding down a key. That key will
-			// remain in the event queue so the next time a window is opened, that
-			// key will be spammed until that particular key msg is sent here again.
-			hr = ImguiSystem()->MessageHandler(hWnd, WM_NULL, wParam, lParam);
+			// Dry run with kill focus msg to clear the keydown state, we have to do
+			// this as the menu's can be closed while still holding down a key. That
+			// key will remain pressed down so the next time a window is opened that
+			// key will be spammed, until that particular key msg is sent here again.
+			hr = ImguiSystem()->MessageHandler(hWnd, WM_KILLFOCUS, wParam, lParam);
 		}
-
-		g_bBlockInput = false;
 	}
 
 	return hr;

src/windows/input.cpp

@@ -19,7 +19,7 @@ static IShowCursor              g_oShowCursor               = nullptr;
 
 ///////////////////////////////////////////////////////////////////////////////
 static POINT                    g_pLastCursorPos              { 0 };
-extern BOOL                     g_bBlockInput               = false;
+extern std::atomic_bool         g_bBlockInput               = false;
 
 //#############################################################################
 // INITIALIZATION

src/windows/input.h

@@ -7,6 +7,6 @@ void Input_Shutdown();
 
 /////////////////////////////////////////////////////////////////////////////
 // Globals
-extern BOOL g_bBlockInput;
+extern std::atomic_bool g_bBlockInput;
 
 /////////////////////////////////////////////////////////////////////////////