Viewport: Win32, GLFW, SDL: Clarified back-ends by using global mouse position direction. GLFW: disabled io.MouseHoveredViewport setting under Mac/Linux. (#1542, #2117) + various comments.

examples/imgui_impl_glfw.cpp

@@ -141,7 +141,7 @@ static bool ImGui_ImplGlfw_Init(GLFWwindow* window, bool install_callbacks, Glfw
     io.BackendFlags |= ImGuiBackendFlags_HasMouseCursors;         // We can honor GetMouseCursor() values (optional)
     io.BackendFlags |= ImGuiBackendFlags_HasSetMousePos;          // We can honor io.WantSetMousePos requests (optional, rarely used)
     io.BackendFlags |= ImGuiBackendFlags_PlatformHasViewports;    // We can create multi-viewports on the Platform side (optional)
-#if GLFW_HAS_GLFW_HOVERED
+#if GLFW_HAS_GLFW_HOVERED && defined(_WIN32)
     io.BackendFlags |= ImGuiBackendFlags_HasMouseHoveredViewport; // We can set io.MouseHoveredViewport correctly (optional, not easy)
 #endif
     io.BackendPlatformName = "imgui_impl_glfw";
@@ -264,14 +264,32 @@ static void ImGui_ImplGlfw_UpdateMousePosAndButtons()
             {
                 double mouse_x, mouse_y;
                 glfwGetCursorPos(window, &mouse_x, &mouse_y);
-                io.MousePos = ImVec2((float)mouse_x + viewport->Pos.x, (float)mouse_y + viewport->Pos.y);
+                if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable)
+                {
+                    // Multi-viewport mode: mouse position in OS absolute coordinates (io.MousePos is (0,0) when the mouse is on the upper-left of the primary monitor)
+                    int window_x, window_y;
+                    glfwGetWindowPos(window, &window_x, &window_y);
+                    io.MousePos = ImVec2((float)mouse_x + window_x, (float)mouse_y + window_y);
+                }
+                else
+                {
+                    // Single viewport mode: mouse position in client window coordinates (io.MousePos is (0,0) when the mouse is on the upper-left corner of the app window)
+                    io.MousePos = ImVec2((float)mouse_x, (float)mouse_y);
+                }
             }
             for (int i = 0; i < IM_ARRAYSIZE(io.MouseDown); i++)
                 io.MouseDown[i] |= glfwGetMouseButton(window, i) != 0;
         }
 
-#if GLFW_HAS_GLFW_HOVERED
+        // (Optional) When using multiple viewports: set io.MouseHoveredViewport to the viewport the OS mouse cursor is hovering.
-        io.BackendFlags |= ImGuiBackendFlags_HasMouseHoveredViewport;
+        // Important: this information is not easy to provide and many high-level windowing library won't be able to provide it correctly, because
+        // - This is _ignoring_ viewports with the ImGuiViewportFlags_NoInputs flag (pass-through windows).
+        // - This is _regardless_ of whether another viewport is focused or being dragged from.
+        // If ImGuiBackendFlags_HasMouseHoveredViewport is not set by the back-end, imgui will ignore this field and infer the information by relying on the
+        // rectangles and last focused time of every viewports it knows about. It will be unaware of other windows that may be sitting between or over your windows.
+        // [GLFW] FIXME: This is currently only correct on Win32. See what we do below with the WM_NCHITTEST, missing an equivalent for other systems.
+        // See https://github.com/glfw/glfw/issues/1236 if you want to help in making this a GLFW feature.
+#if GLFW_HAS_GLFW_HOVERED && defined(_WIN32)
         if (glfwGetWindowAttrib(window, GLFW_HOVERED) && !(viewport->Flags & ImGuiViewportFlags_NoInputs))
             io.MouseHoveredViewport = viewport->ID;
 #endif

examples/imgui_impl_sdl.cpp

@@ -264,16 +264,20 @@ static void ImGui_ImplSDL2_UpdateMousePosAndButtons()
     SDL_Window* focused_window = SDL_GetKeyboardFocus();
     if (focused_window)
     {
-        // SDL_GetMouseState() gives mouse position seemingly based on the last window entered/focused(?)
+        if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable)
-        // The creation of new windows at runtime and SDL_CaptureMouse both seems to severely mess up with that, so we retrieve that position globally.
+        {
-        int wx, wy;
+            // Multi-viewport mode: mouse position in OS absolute coordinates (io.MousePos is (0,0) when the mouse is on the upper-left of the primary monitor)
-        SDL_GetWindowPosition(focused_window, &wx, &wy);
             SDL_GetGlobalMouseState(&mx, &my);
-        mx -= wx;
+            if (ImGui::FindViewportByPlatformHandle((void*)focused_window) != NULL)
-        my -= wy;
+                io.MousePos = ImVec2((float)mx, (float)my);
+        }
+        else
+        {
+            // Single viewport mode: mouse position in client window coordinates (io.MousePos is (0,0) when the mouse is on the upper-left corner of the app window)
+            if (focused_window == g_Window)
+                io.MousePos = ImVec2((float)mx, (float)my);
+        }
     }
-    if (ImGuiViewport* viewport = ImGui::FindViewportByPlatformHandle((void*)focused_window))
-        io.MousePos = ImVec2(viewport->Pos.x + (float)mx, viewport->Pos.y + (float)my);
 
     // We already retrieve global mouse position, SDL_CaptureMouse() also let the OS know e.g. that our imgui drag outside the SDL window boundaries shouldn't trigger the OS window resize cursor
     // The function is only supported from SDL 2.0.4 (released Jan 2016)

examples/imgui_impl_win32.cpp

@@ -135,21 +135,14 @@ static bool ImGui_ImplWin32_UpdateMouseCursor()
     return true;
 }
 
-// This code supports multiple OS Windows mapped into different ImGui viewports, 
+// This code supports multi-viewports (multiple OS Windows mapped into different Dear ImGui viewports)
-// Because of that, it is a little more complicated than your typical single-viewport binding code.
+// Because of that, it is a little more complicated than your typical single-viewport binding code!
-// A) In Single-viewport mode imgui needs:
-//   - io.MousePos ............... mouse position, in client window coordinates (what you'd get from GetCursorPos+ScreenToClient() or from WM_MOUSEMOVE) 
-//                                 io.MousePos is (0,0) when the mouse is on the upper-left corner of the application window.
-// B) In Multi-viewport mode imgui needs: (when ImGuiConfigFlags_ViewportsEnable is set)
-//   - io.MousePos ............... mouse position, in OS absolute coordinates (what you'd get from GetCursorPos(), or from WM_MOUSEMOVE+viewport->Pos). 
-//                                 io.MousePos is (0,0) when the mouse is on the upper-left of the primary monitor.
-//   - io.MouseHoveredViewport ... [optional] viewport which mouse is hovering, with _VERY_ specific and strict conditions (Read comments next to io.MouseHoveredViewport. This is _NOT_ easy to provide in many high-level engine because of how we use the ImGuiViewportFlags_NoInputs flag)
 static void ImGui_ImplWin32_UpdateMousePos()
 {
     ImGuiIO& io = ImGui::GetIO();
 
     // Set OS mouse position if requested (rarely used, only when ImGuiConfigFlags_NavEnableSetMousePos is enabled by user)
-    // (When multi-viewports are enabled, all imgui positions are same as OS positions.)
+    // (When multi-viewports are enabled, all imgui positions are same as OS positions)
     if (io.WantSetMousePos)
     {
         POINT pos = { (int)io.MousePos.x, (int)io.MousePos.y };
@@ -161,21 +154,39 @@ static void ImGui_ImplWin32_UpdateMousePos()
     io.MousePos = ImVec2(-FLT_MAX, -FLT_MAX);
     io.MouseHoveredViewport = 0;
 
-    // Set mouse position and viewport
+    // Set imgui mouse position
-    // (Note that ScreenToClient() and adding +viewport->Pos are mutually cancelling each others when we have multi-viewport enabled. In single-viewport mode, viewport->Pos will be zero)
+    POINT mouse_screen_pos;
-    POINT pos;
+    if (!::GetCursorPos(&mouse_screen_pos))
-    if (!::GetCursorPos(&pos))
         return;
     if (HWND focused_hwnd = ::GetActiveWindow())
-        if (ImGuiViewport* viewport = ImGui::FindViewportByPlatformHandle((void*)focused_hwnd))
     {
-            POINT client_pos = pos;
+        if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable)
-            ::ScreenToClient(focused_hwnd, &client_pos);
+        {
-            io.MousePos = ImVec2(viewport->Pos.x + (float)client_pos.x, viewport->Pos.y + (float)client_pos.y);
+            // Multi-viewport mode: mouse position in OS absolute coordinates (io.MousePos is (0,0) when the mouse is on the upper-left of the primary monitor)
+            // This is the position you can get with GetCursorPos(). In theory adding viewport->Pos is also the reverse operation of doing ScreenToClient().
+            if (ImGui::FindViewportByPlatformHandle((void*)focused_hwnd) != NULL)
+                io.MousePos = ImVec2((float)mouse_screen_pos.x, (float)mouse_screen_pos.y);
+        }
+        else
+        {
+            // Single viewport mode: mouse position in client window coordinates (io.MousePos is (0,0) when the mouse is on the upper-left corner of the app window.)
+            // This is the position you can get with GetCursorPos() + ScreenToClient() or from WM_MOUSEMOVE.
+            if (focused_hwnd == g_hWnd)
+            {
+                POINT mouse_client_pos = mouse_screen_pos;
+                ::ScreenToClient(focused_hwnd, &mouse_client_pos);
+                io.MousePos = ImVec2((float)mouse_client_pos.x, (float)mouse_client_pos.y);
+            }
+        }
     }
 
-    // Our back-end can tell which window is under the mouse cursor (not every back-end can), so pass that info to imgui
+    // (Optional) When using multiple viewports: set io.MouseHoveredViewport to the viewport the OS mouse cursor is hovering.
-    if (HWND hovered_hwnd = ::WindowFromPoint(pos))
+    // Important: this information is not easy to provide and many high-level windowing library won't be able to provide it correctly, because
+    // - This is _ignoring_ viewports with the ImGuiViewportFlags_NoInputs flag (pass-through windows).
+    // - This is _regardless_ of whether another viewport is focused or being dragged from.
+    // If ImGuiBackendFlags_HasMouseHoveredViewport is not set by the back-end, imgui will ignore this field and infer the information by relying on the
+    // rectangles and last focused time of every viewports it knows about. It will be unaware of other windows that may be sitting between or over your windows.
+    if (HWND hovered_hwnd = ::WindowFromPoint(mouse_screen_pos))
         if (ImGuiViewport* viewport = ImGui::FindViewportByPlatformHandle((void*)hovered_hwnd))
             io.MouseHoveredViewport = viewport->ID;
 }

imgui.h

@@ -1231,7 +1231,7 @@ struct ImGuiIO
     bool        MouseDown[5];                   // Mouse buttons: 0=left, 1=right, 2=middle + extras. ImGui itself mostly only uses left button (BeginPopupContext** are using right button). Others buttons allows us to track if the mouse is being used by your application + available to user as a convenience via IsMouse** API.
     float       MouseWheel;                     // Mouse wheel Vertical: 1 unit scrolls about 5 lines text.
     float       MouseWheelH;                    // Mouse wheel Horizontal. Most users don't have a mouse with an horizontal wheel, may not be filled by all back-ends.
-    ImGuiID     MouseHoveredViewport;           // (Optional) When using multiple viewports: viewport the OS mouse cursor is hovering _IGNORING_ viewports with the ImGuiViewportFlags_NoInputs flag, and _REGARDLESS_ of whether another viewport is focused. Set io.BackendFlags |= ImGuiBackendFlags_HasMouseHoveredViewport if you can provide this info. If you don't imgui will use a decent heuristic instead.
+    ImGuiID     MouseHoveredViewport;           // (Optional) When using multiple viewports: viewport the OS mouse cursor is hovering _IGNORING_ viewports with the ImGuiViewportFlags_NoInputs flag, and _REGARDLESS_ of whether another viewport is focused. Set io.BackendFlags |= ImGuiBackendFlags_HasMouseHoveredViewport if you can provide this info. If you don't imgui will infer the value using the rectangles and last focused time of the viewports it knows about (ignoring other OS windows).
     bool        KeyCtrl;                        // Keyboard modifier pressed: Control
     bool        KeyShift;                       // Keyboard modifier pressed: Shift
     bool        KeyAlt;                         // Keyboard modifier pressed: Alt