Backends: SDL: no support for ImGuiBackendFlags_HasMouseHoveredViewport under OSX/LInux (#4960)

backends/imgui_impl_sdl.cpp

@@ -367,10 +367,13 @@ static bool ImGui_ImplSDL2_Init(SDL_Window* window, void* sdl_gl_context)
     io.BackendFlags |= ImGuiBackendFlags_HasMouseCursors;           // We can honor GetMouseCursor() values (optional)
     io.BackendFlags |= ImGuiBackendFlags_HasSetMousePos;            // We can honor io.WantSetMousePos requests (optional, rarely used)
     if (mouse_can_use_global_state)
-    {
         io.BackendFlags |= ImGuiBackendFlags_PlatformHasViewports;  // We can create multi-viewports on the Platform side (optional)
+
+    // SDL on Linux/OSX doesn't report events for unfocused windows (see https://github.com/ocornut/imgui/issues/4960)
+#ifdef _WIN32
+    if (mouse_can_use_global_state)
         io.BackendFlags |= ImGuiBackendFlags_HasMouseHoveredViewport;// We can call io.AddMouseViewportEvent() with correct data (optional)
-    }
+#endif
 
     bd->Window = window;
     bd->MouseCanUseGlobalState = mouse_can_use_global_state;
@@ -528,11 +531,14 @@ static void ImGui_ImplSDL2_UpdateMouseData()
     //       for docking, the viewport has the _NoInputs flag in order to allow us to find the viewport under), then Dear ImGui is forced to ignore the value reported
     //       by the backend, and use its flawed heuristic to guess the viewport behind.
     // - [X] SDL backend correctly reports this regardless of another viewport behind focused and dragged from (we need this to find a useful drag and drop target).
-    ImGuiID mouse_viewport_id = 0;
-    if (SDL_Window* sdl_mouse_window = SDL_GetWindowFromID(bd->MouseWindowID))
-        if (ImGuiViewport* mouse_viewport = ImGui::FindViewportByPlatformHandle((void*)sdl_mouse_window))
-            mouse_viewport_id = mouse_viewport->ID;
-    io.AddMouseViewportEvent(mouse_viewport_id);
+    if (io.BackendFlags & ImGuiBackendFlags_HasMouseHoveredViewport)
+    {
+        ImGuiID mouse_viewport_id = 0;
+        if (SDL_Window* sdl_mouse_window = SDL_GetWindowFromID(bd->MouseWindowID))
+            if (ImGuiViewport* mouse_viewport = ImGui::FindViewportByPlatformHandle((void*)sdl_mouse_window))
+                mouse_viewport_id = mouse_viewport->ID;
+        io.AddMouseViewportEvent(mouse_viewport_id);
+    }
 }
 
 static void ImGui_ImplSDL2_UpdateMouseCursor()