Docking: Fix generalizing the transition from loose windows to dock node so we can switch to single window dock node while transferring viewports (vs creating two viewports and dropping one). + Extracted DockContextGenNodeID out of DockContextAddNode().

imgui.cpp

@@ -9818,6 +9818,7 @@ namespace ImGui
 {
     // ImGuiDockContext
     static ImGuiDockNode*   DockContextAddNode(ImGuiContext* ctx, ImGuiID id);
+    static ImGuiID          DockContextGenNodeID(ImGuiContext* ctx);
     static void             DockContextRemoveNode(ImGuiContext* ctx, ImGuiDockNode* node, bool merge_sibling_into_parent_node);
     static void             DockContextQueueDock(ImGuiContext* ctx, ImGuiWindow* target, ImGuiDockNode* target_node, ImGuiWindow* payload, ImGuiDir split_dir, float split_ratio, bool split_outer);
     static void             DockContextQueueNotifyRemovedNode(ImGuiContext* ctx, ImGuiDockNode* node);
@@ -10008,17 +10009,23 @@ static ImGuiDockNode* ImGui::DockContextFindNodeByID(ImGuiContext* ctx, ImGuiID
     return (ImGuiDockNode*)ctx->DockContext->Nodes.GetVoidPtr(id);
 }
 
+static ImGuiID ImGui::DockContextGenNodeID(ImGuiContext* ctx)
+{
+    // Generate an ID for new node (the exact ID value doesn't matter as long as it is not already used)
+    // FIXME-OPT: This is suboptimal, even if the node count is small enough not to be a worry. We should poke in ctx->Nodes to find a suitable ID faster.
+    ImGuiID id = 0x0001;
+    while (DockContextFindNodeByID(ctx, id) != NULL)
+        id++;
+    return id;
+}
+
 static ImGuiDockNode* ImGui::DockContextAddNode(ImGuiContext* ctx, ImGuiID id)
 {
     // Generate an ID for the new node (the exact ID value doesn't matter as long as it is not already used) and add the first window.
     if (id == 0)
-    {
-        // FIXME-OPT: This is suboptimal, even if the node count is small enough not to be a worry. We could poke in ctx->Nodes to find a suitable ID faster.
-        id = 0x0001;
-        while (DockContextFindNodeByID(ctx, id) != NULL)
-            id++;
-    }
-    IM_ASSERT(DockContextFindNodeByID(ctx, id) == NULL);
+        id = DockContextGenNodeID(ctx);
+    else
+        IM_ASSERT(DockContextFindNodeByID(ctx, id) == NULL);
     ImGuiDockNode* node = IM_NEW(ImGuiDockNode)(id);
     node->InitFromFirstWindowPosSize = node->InitFromFirstWindowViewport = true;
     ctx->DockContext->Nodes.SetVoidPtr(node->ID, node);
@@ -10436,14 +10443,15 @@ static void ImGui::DockNodeAddWindow(ImGuiDockNode* node, ImGuiWindow* window, b
 
     // If 2+ windows appeared on the same frame, creating a new DockNode+TabBar from the second window, 
     // then we need to hide the first one after the fact otherwise it would be visible as a standalone window for one frame.
-    if (node->Windows.Size == 2 && node->HostWindow == NULL && node->Windows[0]->WasActive == false)
+    if (node->HostWindow == NULL && node->Windows.Size == 2 && node->Windows[0]->WasActive == false)
     {
         node->Windows[0]->Hidden = true;
         node->Windows[0]->HiddenFramesRegular = 1;
     }
 
-    // When reactivating a node from two loose window, the target window pos/size are authoritative
-    if (node->Windows.Size == 2 && !node->IsDockSpace)
+    // When reactivating a node with one or two loose window, the window pos/size/viewport are authoritative over the node storage.
+    // In particular it is important we init the viewport from the first window so we don't create two viewports and drop one.
+    if (node->HostWindow == NULL && !node->IsDockSpace && node->IsRootNode())
         node->InitFromFirstWindowPosSize = node->InitFromFirstWindowViewport = true;
 
     // Add to tab bar if requested
@@ -10817,7 +10825,6 @@ static void ImGui::DockNodeUpdate(ImGuiDockNode* node)
             }
             else if (node->HostWindow == NULL)
             {
-                // Leaf
                 SetNextWindowPos(node->Pos);
                 SetNextWindowSize(node->Size);
             }