Backends: Win32, SDL, GLFW: only honor io.WantSetMousePos when focused + fix GLFW uninstalling handler + tweaks to reduce branch drift with docking. (#787, #2445, #2696, #3751, #4377)

# Conflicts:
#	backends/imgui_impl_glfw.cpp
#	backends/imgui_impl_sdl.cpp
#	backends/imgui_impl_win32.cpp

backends/imgui_impl_glfw.cpp

@@ -369,7 +369,7 @@ static void ImGui_ImplGlfw_UpdateMousePosAndButtons()
     io.MousePos = ImVec2(-FLT_MAX, -FLT_MAX);
     io.MouseHoveredViewport = 0;
 
-    // Update mouse button
+    // Update mouse buttons
     // (if a mouse press event came, always pass it as "mouse held this frame", so we don't miss click-release events that are shorter than 1 frame)
     for (int i = 0; i < IM_ARRAYSIZE(io.MouseDown); i++)
     {
@@ -384,10 +384,11 @@ static void ImGui_ImplGlfw_UpdateMousePosAndButtons()
 
 #ifdef __EMSCRIPTEN__
         const bool focused = true;
-        IM_ASSERT(platform_io.Viewports.Size == 1);
 #else
         const bool focused = glfwGetWindowAttrib(window, GLFW_FOCUSED) != 0;
 #endif
+        GLFWwindow* mouse_window = (bd->MouseWindow == window || focused) ? window : NULL;
+
         // Update mouse buttons
         if (focused)
             for (int i = 0; i < IM_ARRAYSIZE(io.MouseDown); i++)
@@ -399,10 +400,10 @@ static void ImGui_ImplGlfw_UpdateMousePosAndButtons()
             glfwSetCursorPos(window, (double)(mouse_pos_prev.x - viewport->Pos.x), (double)(mouse_pos_prev.y - viewport->Pos.y));
 
         // Set Dear ImGui mouse position from OS position
-        if (bd->MouseWindow == window || focused)
+        if (mouse_window != NULL)
         {
             double mouse_x, mouse_y;
-            glfwGetCursorPos(window, &mouse_x, &mouse_y);
+            glfwGetCursorPos(mouse_window, &mouse_x, &mouse_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)

backends/imgui_impl_sdl.cpp

@@ -254,7 +254,7 @@ static bool ImGui_ImplSDL2_Init(SDL_Window* window, void* sdl_gl_context)
     // Our mouse update function expect PlatformHandle to be filled for the main viewport
     ImGuiViewport* main_viewport = ImGui::GetMainViewport();
     main_viewport->PlatformHandle = (void*)window;
-#if defined(_WIN32)
+#ifdef _WIN32
     SDL_SysWMinfo info;
     SDL_VERSION(&info.version);
     if (SDL_GetWindowWMInfo(window, &info))
@@ -318,7 +318,6 @@ void ImGui_ImplSDL2_Shutdown()
 
     ImGui_ImplSDL2_ShutdownPlatformInterface();
 
-    // Destroy last known clipboard data
     if (bd->ClipboardTextData)
         SDL_free(bd->ClipboardTextData);
     for (ImGuiMouseCursor cursor_n = 0; cursor_n < ImGuiMouseCursor_COUNT; cursor_n++)
@@ -332,12 +331,12 @@ void ImGui_ImplSDL2_Shutdown()
 // This code is incredibly messy because some of the functions we need for full viewport support are not available in SDL < 2.0.4.
 static void ImGui_ImplSDL2_UpdateMousePosAndButtons()
 {
-    ImGuiIO& io = ImGui::GetIO();
     ImGui_ImplSDL2_Data* bd = ImGui_ImplSDL2_GetBackendData();
+    ImGuiIO& io = ImGui::GetIO();
 
     ImVec2 mouse_pos_prev = io.MousePos;
-    io.MouseHoveredViewport = 0;
     io.MousePos = ImVec2(-FLT_MAX, -FLT_MAX);
+    io.MouseHoveredViewport = 0;
 
     // Update mouse buttons
     int mouse_x_local, mouse_y_local;
@@ -391,7 +390,7 @@ static void ImGui_ImplSDL2_UpdateMousePosAndButtons()
         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)
-            // Unlike local position obtained earlier this will be valid when straying out of bounds too.
+            // Unlike local position obtained earlier this will be valid when straying out of bounds.
             int window_x, window_y;
             SDL_GetWindowPosition(mouse_window, &window_x, &window_y);
             io.MousePos = ImVec2((float)(mouse_x_global - window_x), (float)(mouse_y_global - window_y));

backends/imgui_impl_win32.cpp

@@ -72,7 +72,6 @@ static void ImGui_ImplWin32_InitPlatformInterface();
 static void ImGui_ImplWin32_ShutdownPlatformInterface();
 static void ImGui_ImplWin32_UpdateMonitors();
 
-// Win32
 struct ImGui_ImplWin32_Data
 {
     HWND                        hWnd;
@@ -259,7 +258,7 @@ static void ImGui_ImplWin32_UpdateMousePos()
 
     // Set OS mouse position from Dear ImGui if requested (rarely used, only when ImGuiConfigFlags_NavEnableSetMousePos is enabled by user)
     // (When multi-viewports are enabled, all Dear ImGui positions are same as OS positions)
-    if (io.WantSetMousePos && mouse_window != NULL)
+    if (io.WantSetMousePos)
     {
         POINT pos = { (int)mouse_pos_prev.x, (int)mouse_pos_prev.y };
         if ((io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable) == 0)