Docking+Viewports: Fixed extraneous viewport+platform-window recreation. Part 1.

Part 1: Add counters. Amend logs.
Refer to "viewport_owner_change_1" and "viewport_owner_change_2" in ImGuiTestSuite.

docs/CHANGELOG.txt

@@ -151,6 +151,11 @@ Other changes:
 
 Docking+Viewports Branch:
 
+- Viewports+Docking: Fixed extraneous viewport+platform-window recreation in various
+  combination of showing or hiding windows, docking with/without split, undocking.
+  While with some backends and without OS decorations, some extraneous window recreation
+  were visibly not noticeable, they would typically become noticeable when enabling
+  OS decorations on those windows (e.g. Windows title bar fade-in/animation).
 - Viewports: Closing a viewport via OS/platform means (e.g. OS close button or task-bar menu),
   mark all windows in this viewport as closed.
 - Docking: Fixed one-frame flickering on reappearing windows binding to a dock node

imgui.cpp

@@ -3631,6 +3631,7 @@ void ImGui::Initialize()
     viewport->Flags = ImGuiViewportFlags_OwnedByApp;
     g.Viewports.push_back(viewport);
     g.TempBuffer.resize(1024 * 3 + 1, 0);
+    g.ViewportCreatedCount++;
     g.PlatformIO.Viewports.push_back(g.Viewports[0]);
 
 #ifdef IMGUI_HAS_DOCK
@@ -14373,6 +14374,7 @@ ImGuiViewportP* ImGui::AddUpdateViewport(ImGuiWindow* window, ImGuiID id, const
         viewport->Flags = flags;
         UpdateViewportPlatformMonitor(viewport);
         g.Viewports.push_back(viewport);
+        g.ViewportCreatedCount++;
         IMGUI_DEBUG_LOG_VIEWPORT("[viewport] Add Viewport %08X '%s'\n", id, window ? window->Name : "<NULL>");
 
         // We normally setup for all viewports in NewFrame() but here need to handle the mid-frame creation of a new viewport.
@@ -14685,6 +14687,7 @@ void ImGui::UpdatePlatformWindows()
             g.PlatformIO.Platform_CreateWindow(viewport);
             if (g.PlatformIO.Renderer_CreateWindow != NULL)
                 g.PlatformIO.Renderer_CreateWindow(viewport);
+            g.PlatformWindowsCreatedCount++;
             viewport->LastNameHash = 0;
             viewport->LastPlatformPos = viewport->LastPlatformSize = ImVec2(FLT_MAX, FLT_MAX); // By clearing those we'll enforce a call to Platform_SetWindowPos/Size below, before Platform_ShowWindow (FIXME: Is that necessary?)
             viewport->LastRendererSize = viewport->Size;                                       // We don't need to call Renderer_SetWindowSize() as it is expected Renderer_CreateWindow() already did it.
@@ -16254,6 +16257,7 @@ static void ImGui::DockNodeUpdate(ImGuiDockNode* node)
                 FocusWindow(single_window);
             if (node->HostWindow)
             {
+                IMGUI_DEBUG_LOG_VIEWPORT("[viewport] Node %08X transfer Viewport %08X->%08X to Window '%s'\n", node->ID, node->HostWindow->Viewport->ID, single_window->ID, single_window->Name);
                 single_window->Viewport = node->HostWindow->Viewport;
                 single_window->ViewportId = node->HostWindow->ViewportId;
                 if (node->HostWindow->ViewportOwned)

imgui_internal.h

@@ -2058,6 +2058,8 @@ struct ImGuiContext
     ImGuiViewportP*         MouseLastHoveredViewport;           // Last known viewport that was hovered by mouse (even if we are not hovering any viewport any more) + honoring the _NoInputs flag.
     ImGuiID                 PlatformLastFocusedViewportId;
     ImGuiPlatformMonitor    FallbackMonitor;                    // Virtual monitor used as fallback if backend doesn't provide monitor information.
+    int                     ViewportCreatedCount;               // Unique sequential creation counter (mostly for testing/debugging)
+    int                     PlatformWindowsCreatedCount;        // Unique sequential creation counter (mostly for testing/debugging)
     int                     ViewportFocusedStampCount;          // Every time the front-most window changes, we stamp its viewport with an incrementing counter
 
     // Gamepad/keyboard Navigation
@@ -2327,6 +2329,7 @@ struct ImGuiContext
         CurrentViewport = NULL;
         MouseViewport = MouseLastHoveredViewport = NULL;
         PlatformLastFocusedViewportId = 0;
+        ViewportCreatedCount = PlatformWindowsCreatedCount = 0;
         ViewportFocusedStampCount = 0;
 
         NavWindow = NULL;