From 5d74891285d97fe4160690389296d1448aa24afa Mon Sep 17 00:00:00 2001
From: ocornut <omarcornut@gmail.com>
Date: Wed, 15 Feb 2023 17:17:34 +0100
Subject: [PATCH] Backends: Win32: Use WM_NCMOUSEMOVE / WM_NCMOUSELEAVE to
 track mouse positions over non-client area (OS decorations) when app is not
 focused. (#6045, #6162)

---
 backends/imgui_impl_win32.cpp | 36 +++++++++++++++++++++++++----------
 docs/CHANGELOG.txt            | 10 +++++++++-
 2 files changed, 35 insertions(+), 11 deletions(-)

diff --git a/backends/imgui_impl_win32.cpp b/backends/imgui_impl_win32.cpp
index 7ebb58c56..7cc571788 100644
--- a/backends/imgui_impl_win32.cpp
+++ b/backends/imgui_impl_win32.cpp
@@ -34,6 +34,7 @@ typedef DWORD (WINAPI *PFN_XInputGetState)(DWORD, XINPUT_STATE*);
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2023-02-15: Inputs: Use WM_NCMOUSEMOVE / WM_NCMOUSELEAVE to track mouse position over non-client area (e.g. OS decorations) when app is not focused. (#6045, #6162)
 //  2023-02-02: Inputs: Flipping WM_MOUSEHWHEEL (horizontal mouse-wheel) value to match other backends and offer consistent horizontal scrolling direction. (#4019, #6096, #1463)
 //  2022-10-11: Using 'nullptr' instead of 'NULL' as per our switch to C++11.
 //  2022-09-28: Inputs: Convert WM_CHAR values with MultiByteToWideChar() when window class was registered as MBCS (not Unicode).
@@ -84,7 +85,7 @@ struct ImGui_ImplWin32_Data
 {
     HWND                        hWnd;
     HWND                        MouseHwnd;
-    bool                        MouseTracked;
+    int                         MouseTrackedArea;   // 0: not tracked, 1: client are, 2: non-client area
     int                         MouseButtonsDown;
     INT64                       Time;
     INT64                       TicksPerSecond;
@@ -267,7 +268,8 @@ static void ImGui_ImplWin32_UpdateMouseData()
         }
 
         // (Optional) Fallback to provide mouse position when focused (WM_MOUSEMOVE already provides this when hovered or captured)
-        if (!io.WantSetMousePos && !bd->MouseTracked)
+        // This also fills a short gap when clicking non-client area: WM_NCMOUSELEAVE -> modal OS move -> gap -> WM_NCMOUSEMOVE
+        if (!io.WantSetMousePos && bd->MouseTrackedArea == 0)
         {
             POINT pos;
             if (::GetCursorPos(&pos) && ::ScreenToClient(bd->hWnd, &pos))
@@ -515,25 +517,39 @@ IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hwnd, UINT msg, WPARA
     switch (msg)
     {
     case WM_MOUSEMOVE:
+    case WM_NCMOUSEMOVE:
     {
         // We need to call TrackMouseEvent in order to receive WM_MOUSELEAVE events
+        const int area = (msg == WM_MOUSEMOVE) ? 1 : 2;
         bd->MouseHwnd = hwnd;
-        if (!bd->MouseTracked)
+        if (bd->MouseTrackedArea != area)
         {
-            TRACKMOUSEEVENT tme = { sizeof(tme), TME_LEAVE, hwnd, 0 };
-            ::TrackMouseEvent(&tme);
-            bd->MouseTracked = true;
+            TRACKMOUSEEVENT tme_cancel = { sizeof(tme_cancel), TME_CANCEL, hwnd, 0 };
+            TRACKMOUSEEVENT tme_track = { sizeof(tme_track), (DWORD)((area == 2) ? (TME_LEAVE | TME_NONCLIENT) : TME_LEAVE), hwnd, 0 };
+            if (bd->MouseTrackedArea != 0)
+                ::TrackMouseEvent(&tme_cancel);
+            ::TrackMouseEvent(&tme_track);
+            bd->MouseTrackedArea = area;
         }
         POINT mouse_pos = { (LONG)GET_X_LPARAM(lParam), (LONG)GET_Y_LPARAM(lParam) };
+        if (msg == WM_NCMOUSEMOVE && ::ScreenToClient(hwnd, &mouse_pos) == FALSE) // WM_NCMOUSEMOVE are provided in absolute coordinates.
+            break;
         io.AddMousePosEvent((float)mouse_pos.x, (float)mouse_pos.y);
         break;
     }
     case WM_MOUSELEAVE:
-        if (bd->MouseHwnd == hwnd)
-            bd->MouseHwnd = nullptr;
-        bd->MouseTracked = false;
-        io.AddMousePosEvent(-FLT_MAX, -FLT_MAX);
+    case WM_NCMOUSELEAVE:
+    {
+        const int area = (msg == WM_MOUSELEAVE) ? 1 : 2;
+        if (bd->MouseTrackedArea == area)
+        {
+            if (bd->MouseHwnd == hwnd)
+                bd->MouseHwnd = nullptr;
+            bd->MouseTrackedArea = 0;
+            io.AddMousePosEvent(-FLT_MAX, -FLT_MAX);
+        }
         break;
+    }
     case WM_LBUTTONDOWN: case WM_LBUTTONDBLCLK:
     case WM_RBUTTONDOWN: case WM_RBUTTONDBLCLK:
     case WM_MBUTTONDOWN: case WM_MBUTTONDBLCLK:
diff --git a/docs/CHANGELOG.txt b/docs/CHANGELOG.txt
index 09732689e..642a1ac3e 100644
--- a/docs/CHANGELOG.txt
+++ b/docs/CHANGELOG.txt
@@ -35,6 +35,14 @@ HOW TO UPDATE?
  VERSION 1.89.4 WIP (In Progress)
 -----------------------------------------------------------------------
 
+Breaking Changes:
+
+Other changes:
+
+- Backends: Win32: Use WM_NCMOUSEMOVE / WM_NCMOUSELEAVE to track mouse positions over
+  non-client area (e.g. OS decorations) when app is not focused. (#6045, #6162)
+
+
 
 -----------------------------------------------------------------------
  VERSION 1.89.3 (Released 2023-02-14)
@@ -50,7 +58,7 @@ Breaking Changes:
   - imgui_impl_sdl.h   -> imgui_impl_sdl2.h
   - example_sdl_xxxx/  -> example_sdl2_xxxx/ (folders and projects)
 
-All changes:
+Other changes:
 
 - SeparatorText(): Added SeparatorText() widget. (#1643) [@phed, @ocornut]
   - Added to style: float  SeparatorTextBorderSize.