Viewport: When resizing/moving a window using the host OS/WM we attempt to merge back into host viewport. (#1542)

imgui.cpp

@@ -3553,7 +3553,7 @@ static void ImGui::UpdateViewports()
         if (viewport->PlatformRequestResize)
             viewport->Size = g.PlatformIO.Platform_GetWindowSize(viewport);
 
-        // Translate imgui windows when a host viewport has been moved
+        // Translate imgui windows when a Host Viewport has been moved
         ImVec2 delta = viewport->Pos - viewport->LastPos;
         if ((viewport->Flags & ImGuiViewportFlags_CanHostOtherWindows) && (delta.x != 0.0f || delta.y != 0.0f))
             for (int window_n = 0; window_n < g.Windows.Size; window_n++)
@@ -6827,13 +6827,19 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
             }
         }
 
-        // Synchronize viewport --> window
+        // Synchronize viewport --> window in case the platform window has been moved or resized from the OS/WM
         if (window->ViewportOwned)
         {
             if (window->Viewport->PlatformRequestMove)
+            {
                 window->Pos = window->Viewport->Pos;
+                window->ViewportTryMerge = true;
+            }
             if (window->Viewport->PlatformRequestResize)
+            {
                 window->Size = window->SizeFull = window->Viewport->Size;
+                window->ViewportTryMerge = true;
+            }
 
             // We also tell the back-end that clearing the platform window won't be necessary, as our window is filling the viewport and we have disabled BgAlpha
             window->Viewport->Flags |= ImGuiViewportFlags_NoRendererClear;

imgui.h

@@ -2004,8 +2004,8 @@ struct ImGuiViewport
     void*               PlatformUserData;       // void* to hold custom data structure for the platform (e.g. windowing info, render context)
     void*               PlatformHandle;         // void* for FindViewportByPlatformHandle(). (e.g. suggested to use natural platform handle such as HWND, GlfwWindow*, SDL_Window*)
     bool                PlatformRequestClose;   // Platform window requested closure
-    bool                PlatformRequestMove;    // Platform window requested move (e.g. window was moved using OS windowing facility)
-    bool                PlatformRequestResize;  // Platform window requested resize (e.g. window was resize using OS windowing facility)
+    bool                PlatformRequestMove;    // Platform window requested move (e.g. window was moved by the OS / host window manager)
+    bool                PlatformRequestResize;  // Platform window requested resize (e.g. window was resize by the OS / host window manager)
     void*               RendererUserData;       // void* to hold custom data structure for the renderer (e.g. swap chain, frame-buffers etc.)
 
     ImGuiViewport()     { ID = 0; Flags = 0; DpiScale = 0.0f; DrawData = NULL; PlatformUserData = PlatformHandle = NULL; PlatformRequestClose = PlatformRequestMove = PlatformRequestResize = false; RendererUserData = NULL; }