Viewport: Disable extra viewports from hosting other windows. (#1542)

examples/vulkan_example/main.cpp

@@ -392,7 +392,6 @@ int main(int, char**)
         VkCommandPool command_pool = wd->Frames[wd->FrameIndex].CommandPool;
         VkCommandBuffer command_buffer = wd->Frames[wd->FrameIndex].CommandBuffer;
 
-        VkResult err;
         err = vkResetCommandPool(g_Device, command_pool, 0);
         check_vk_result(err);
         VkCommandBufferBeginInfo begin_info = {};

imgui.cpp

@@ -3252,7 +3252,7 @@ static void ImGui::UpdateMovingWindowDropViewport(ImGuiWindow* window)
     ImRect mouse_viewport_rect = g.MouseViewport->GetRect();
     ImVec2 window_pos_in_mouse_viewport = ConvertOsDesktopPosToViewportPos(ConvertViewportPosToOsDesktopPos(window->Pos, window->Viewport), g.MouseViewport);
     ImRect window_rect_in_mouse_viewport = ImRect(window_pos_in_mouse_viewport, window_pos_in_mouse_viewport + window->Size);
-    if (mouse_viewport_rect.Contains(window_rect_in_mouse_viewport))
+    if ((g.MouseViewport->Flags & ImGuiViewportFlags_HostOtherWindows) &&  mouse_viewport_rect.Contains(window_rect_in_mouse_viewport))
     {
         // Drop on an existing viewport
         ImGuiViewport* old_viewport = window->Viewport;
@@ -3388,7 +3388,7 @@ static void ImGui::UpdateViewports()
     ImVec2 main_viewport_os_desktop_pos = ImVec2(0.0f, 0.0f);
     if ((g.IO.ConfigFlags & ImGuiConfigFlags_EnableViewports))
         main_viewport_os_desktop_pos = g.IO.PlatformInterface.GetWindowPos(main_viewport);
-    Viewport(IMGUI_VIEWPORT_DEFAULT_ID, ImGuiViewportFlags_MainViewport, main_viewport_os_desktop_pos, g.IO.DisplaySize);
+    Viewport(IMGUI_VIEWPORT_DEFAULT_ID, ImGuiViewportFlags_MainViewport | ImGuiViewportFlags_HostOtherWindows, main_viewport_os_desktop_pos, g.IO.DisplaySize);
 
     if (!(g.IO.ConfigFlags & ImGuiConfigFlags_EnableViewports))
     {

imgui_internal.h

@@ -517,9 +517,10 @@ struct ImDrawDataBuilder
 enum ImGuiViewportFlags_
 {
     ImGuiViewportFlags_MainViewport         = 1 << 0,
-    ImGuiViewportFlags_NoDecoration         = 1 << 1,   // Platform Window: Disable platform title bar, borders, etc.
-    ImGuiViewportFlags_NoFocusOnAppearing   = 1 << 2,   // Platform Window: Don't take focus when created.
-    ImGuiViewportFlags_NoInputs             = 1 << 3    // Platform Window: Make mouse pass through so we can drag this window while peaking behind it.
+    ImGuiViewportFlags_HostOtherWindows     = 1 << 1,
+    ImGuiViewportFlags_NoDecoration         = 1 << 2,   // Platform Window: Disable platform title bar, borders, etc.
+    ImGuiViewportFlags_NoFocusOnAppearing   = 1 << 3,   // Platform Window: Don't take focus when created.
+    ImGuiViewportFlags_NoInputs             = 1 << 4    // Platform Window: Make mouse pass through so we can drag this window while peaking behind it.
 };
 
 struct ImGuiViewport