Viewports: (breaking) removed ImGuiPlatformIO::MainViewport which is now pretty much unused and duplicate (and misleading as we will evolve the concept)

Use GetMainViewport() if stuck.

docs/CHANGELOG.txt

@@ -76,7 +76,10 @@ Other changes:
 (FIXME: This need a fuller explanation!)
 
 - Added ImGuiPlatformIO structure and GetPlatformIO().
-  Similarly to ImGuiIO and GetIO(), this structure is the main point of communication for backends supporting multi-viewports.
+  - Similarly to ImGuiIO and GetIO(), this structure is the main point of communication for backends supporting multi-viewports.
+  - Backend sets functions in ImGuiPlatformIO to manipulate platform windows.
+  - ImGuiPlatformIO::Monitors is a list of platform monitors (input from backend)
+  - ImGuiPlatformIO::Viewports is a list of viewports (output from dear imgui)
 - Added ImGuiPlatformMonitor to feed OS monitor information in the ImGuiPlatformIO::Monitors.
 - Added GetMainViewport().
 - Added GetWindowViewport(), SetNextWindowViewport().
@@ -88,7 +91,7 @@ Other changes:
 - Added ImGuiConfigFlags_ViewportsEnable configuration flag and other viewport options.
 - Added io.ConfigViewportsNoAutoMerge, io.ConfigViewportsNoTaskBarIcon, io.ConfigViewportsNoDecoration, io.ConfigViewportsNoDefaultParent options.
 - Added ImGuiBackendFlags_PlatformHasViewports, ImGuiBackendFlags_RendererHasViewports, ImGuiBackendFlags_HasMouseHoveredViewport backend flags.
-- Added io.MainViewport, io.Viewports, io.MouseHoveredViewport (MouseHoveredViewport is optional _even_ for multi-viewport support).
+- Added io.MouseHoveredViewport (optional _even_ for multi-viewport support, tied to ImGuiBackendFlags_HasMouseHoveredViewport flag).
 - Added ImGuiViewport structure, ImGuiViewportFlags flags.
 - Added ImGuiWindowClass and SetNextWindowClass() for passing viewport related hints to the OS/platform back-end.
 - Examples: Renderer: OpenGL2, OpenGL3, DirectX11, DirectX12, Vulkan: Added support for multi-viewports.

imgui.cpp

@@ -4220,7 +4220,6 @@ void ImGui::Initialize(ImGuiContext* context)
     viewport->Idx = 0;
     viewport->PlatformWindowCreated = true;
     g.Viewports.push_back(viewport);
-    g.PlatformIO.MainViewport = g.Viewports[0]; // Make it accessible in public-facing GetPlatformIO() immediately (before the first call to EndFrame)
     g.PlatformIO.Viewports.push_back(g.Viewports[0]);
 
     // Extensions
@@ -6413,7 +6412,8 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
                 if (window->Viewport->PlatformMonitor == -1)
                 {
                     // Fallback for "lost" window (e.g. a monitor disconnected): we move the window back over the main viewport
-                    SetWindowPos(window, g.Viewports[0]->Pos + style.DisplayWindowPadding, ImGuiCond_Always);
+                    ImGuiViewport* main_viewport = GetMainViewport();
+                    SetWindowPos(window, main_viewport->Pos + style.DisplayWindowPadding, ImGuiCond_Always);
                 }
                 else
                 {
@@ -11260,13 +11260,14 @@ static bool ImGui::UpdateTryMergeWindowIntoHostViewport(ImGuiWindow* window, ImG
     return true;
 }
 
+// FIXME: handle 0 to N host viewports
 static bool ImGui::UpdateTryMergeWindowIntoHostViewports(ImGuiWindow* window)
 {
     ImGuiContext& g = *GImGui;
     return UpdateTryMergeWindowIntoHostViewport(window, g.Viewports[0]);
 }
 
-// Translate imgui windows when a Host Viewport has been moved
+// Translate Dear ImGui windows when a Host Viewport has been moved
 // (This additionally keeps windows at the same place when ImGuiConfigFlags_ViewportsEnable is toggled!)
 void ImGui::TranslateWindowsInViewport(ImGuiViewportP* viewport, const ImVec2& old_pos, const ImVec2& new_pos)
 {
@@ -11502,7 +11503,6 @@ static void ImGui::UpdateViewportsNewFrame()
 static void ImGui::UpdateViewportsEndFrame()
 {
     ImGuiContext& g = *GImGui;
-    g.PlatformIO.MainViewport = g.Viewports[0];
     g.PlatformIO.Viewports.resize(0);
     for (int i = 0; i < g.Viewports.Size; i++)
     {
@@ -11589,7 +11589,7 @@ static void ImGui::UpdateSelectWindowViewport(ImGuiWindow* window)
     window->ViewportAllowPlatformMonitorExtend = -1;
 
     // Restore main viewport if multi-viewport is not supported by the backend
-    ImGuiViewportP* main_viewport = g.Viewports[0];
+    ImGuiViewportP* main_viewport = (ImGuiViewportP*)GetMainViewport();
     if (!(g.ConfigFlagsCurrFrame & ImGuiConfigFlags_ViewportsEnable))
     {
         SetWindowViewport(window, main_viewport);

imgui.h

@@ -894,7 +894,7 @@ namespace ImGui
     // Read comments around the ImGuiPlatformIO structure for more details.
     // Note: You may use GetWindowViewport() to get the current viewport of the current window.
     IMGUI_API ImGuiPlatformIO&  GetPlatformIO();                                                // platform/renderer functions, for backend to setup + viewports list.
-    IMGUI_API ImGuiViewport*    GetMainViewport();                                              // main viewport. same as GetPlatformIO().MainViewport == GetPlatformIO().Viewports[0].
+    IMGUI_API ImGuiViewport*    GetMainViewport();                                              // return primary/default viewport. In the future in "no main platform window" mode we will direct this to primary monitor.
     IMGUI_API void              UpdatePlatformWindows();                                        // call in main loop. will call CreateWindow/ResizeWindow/etc. platform functions for each secondary viewport, and DestroyWindow for each inactive viewport.
     IMGUI_API void              RenderPlatformWindowsDefault(void* platform_render_arg = NULL, void* renderer_render_arg = NULL); // call in main loop. will call RenderWindow/SwapBuffers platform functions for each secondary viewport which doesn't have the ImGuiViewportFlags_Minimized flag set. May be reimplemented by user for custom rendering needs.
     IMGUI_API void              DestroyPlatformWindows();                                       // call DestroyWindow platform functions for all viewports. call from backend Shutdown() if you need to close platform windows before imgui shutdown. otherwise will be called by DestroyContext().
@@ -2930,7 +2930,6 @@ struct ImGuiPlatformIO
 
     // Viewports list (the list is updated by calling ImGui::EndFrame or ImGui::Render)
     // (in the future we will attempt to organize this feature to remove the need for a "main viewport")
-    ImGuiViewport*                  MainViewport;                           // Guaranteed to be == Viewports[0]
     ImVector<ImGuiViewport*>        Viewports;                              // Main viewports, followed by all secondary viewports.
     ImGuiPlatformIO()               { memset(this, 0, sizeof(*this)); }     // Zero clear
 };
@@ -2973,8 +2972,8 @@ struct ImGuiViewport
     ImVec2              WorkOffsetMin;          // Work Area: Offset from Pos to top-left corner of Work Area. Generally (0,0) or (0,+main_menu_bar_height). Work Area is Full Area but without menu-bars/status-bars (so WorkArea always fit inside Pos/Size!)
     ImVec2              WorkOffsetMax;          // Work Area: Offset from Pos+Size to bottom-right corner of Work Area. Generally (0,0) or (0,-status_bar_height).
     float               DpiScale;               // 1.0f = 96 DPI = No extra scale.
-    ImDrawData*         DrawData;               // The ImDrawData corresponding to this viewport. Valid after Render() and until the next call to NewFrame().
     ImGuiID             ParentViewportId;       // (Advanced) 0: no parent. Instruct the platform backend to setup a parent/child relationship between platform windows.
+    ImDrawData*         DrawData;               // The ImDrawData corresponding to this viewport. Valid after Render() and until the next call to NewFrame().
 
     // Our design separate the Renderer and Platform backends to facilitate combining default backends with each others.
     // When our create your own backend for a custom engine, it is possible that both Renderer and Platform will be handled
@@ -2988,7 +2987,7 @@ struct ImGuiViewport
     bool                PlatformRequestResize;  // Platform window requested resize (e.g. window was resized by the OS / host window manager, authoritative size will be OS window size)
     bool                PlatformRequestClose;   // Platform window requested closure (e.g. window was moved by the OS / host window manager, e.g. pressing ALT-F4)
 
-    ImGuiViewport()     { ID = 0; Flags = 0; DpiScale = 0.0f; DrawData = NULL; ParentViewportId = 0; RendererUserData = PlatformUserData = PlatformHandle = PlatformHandleRaw = NULL; PlatformRequestMove = PlatformRequestResize = PlatformRequestClose = false; }
+    ImGuiViewport()     { memset(this, 0, sizeof(*this)); }
     ~ImGuiViewport()    { IM_ASSERT(PlatformUserData == NULL && RendererUserData == NULL); }
 
     // Access work-area rectangle with GetWorkXXX functions (see comments above)

imgui_widgets.cpp

@@ -6575,7 +6575,7 @@ void ImGui::EndMenuBar()
 bool ImGui::BeginMainMenuBar()
 {
     ImGuiContext& g = *GImGui;
-    ImGuiViewportP* viewport = g.Viewports[0];
+    ImGuiViewportP* viewport = (ImGuiViewportP*)GetMainViewport();
     ImGuiWindow* menu_bar_window = FindWindowByName("##MainMenuBar");
 
     // For the main menu bar, which cannot be moved, we honor g.Style.DisplaySafeAreaPadding to ensure text can be visible on a TV set.