Method DockNodeUpdateForRootNode

imgui_tiny_app/imgui/imgui.cpp:18956–18991

Update CentralNode, OnlyNodeWithWindows, LastFocusedNodeID. Copy window class.

// Update CentralNode, OnlyNodeWithWindows, LastFocusedNodeID. Copy window class.
static void ImGui::DockNodeUpdateForRootNode(ImGuiDockNode* node)
{
    DockNodeUpdateFlagsAndCollapse(node);

    // - Setup central node pointers
    // - Find if there's only a single visible window in the hierarchy (in which case we need to display a regular title bar -> FIXME-DOCK: that last part is not done yet!)
    // Cannot merge this with DockNodeUpdateFlagsAndCollapse() because FirstNodeWithWindows is found after window removal and child collapsing
    ImGuiDockNodeTreeInfo info;
    DockNodeFindInfo(node, &info);
    node->CentralNode = info.CentralNode;
    node->OnlyNodeWithWindows = (info.CountNodesWithWindows == 1) ? info.FirstNodeWithWindows : NULL;
    node->CountNodeWithWindows = info.CountNodesWithWindows;
    if (node->LastFocusedNodeId == 0 && info.FirstNodeWithWindows != NULL)
        node->LastFocusedNodeId = info.FirstNodeWithWindows->ID;

    // Copy the window class from of our first window so it can be used for proper dock filtering.
    // When node has mixed windows, prioritize the class with the most constraint (DockingAllowUnclassed = false) as the reference to copy.
    // FIXME-DOCK: We don't recurse properly, this code could be reworked to work from DockNodeUpdateScanRec.
    if (ImGuiDockNode* first_node_with_windows = info.FirstNodeWithWindows)
    {
        node->WindowClass = first_node_with_windows->Windows[0]->WindowClass;
        for (int n = 1; n < first_node_with_windows->Windows.Size; n++)
            if (first_node_with_windows->Windows[n]->WindowClass.DockingAllowUnclassed == false)
            {
                node->WindowClass = first_node_with_windows->Windows[n]->WindowClass;
                break;
            }
    }

    ImGuiDockNode* mark_node = node->CentralNode;
    while (mark_node)
    {
        mark_node->HasCentralNodeChild = true;
        mark_node = mark_node->ParentNode;
    }
}

static void DockNodeSetupHostWindow(ImGuiDockNode* node, ImGuiWindow* host_window)
{