Method DockNodePreviewDockSetup

imgui_tiny_app/imgui/imgui.cpp:19884–19970

host_node may be NULL if the window doesn't have a DockNode already. FIXME-DOCK: This is misnamed since it's also doing the filtering.

// host_node may be NULL if the window doesn't have a DockNode already.
// FIXME-DOCK: This is misnamed since it's also doing the filtering.
static void ImGui::DockNodePreviewDockSetup(ImGuiWindow* host_window, ImGuiDockNode* host_node, ImGuiWindow* payload_window, ImGuiDockNode* payload_node, ImGuiDockPreviewData* data, bool is_explicit_target, bool is_outer_docking)
{
    ImGuiContext& g = *GImGui;

    // There is an edge case when docking into a dockspace which only has inactive nodes.
    // In this case DockNodeTreeFindNodeByPos() will have selected a leaf node which is inactive.
    // Because the inactive leaf node doesn't have proper pos/size yet, we'll use the root node as reference.
    if (payload_node == NULL)
        payload_node = payload_window->DockNodeAsHost;
    ImGuiDockNode* ref_node_for_rect = (host_node && !host_node->IsVisible) ? DockNodeGetRootNode(host_node) : host_node;
    if (ref_node_for_rect)
        IM_ASSERT(ref_node_for_rect->IsVisible == true);

    // Filter, figure out where we are allowed to dock
    ImGuiDockNodeFlags src_node_flags = payload_node ? payload_node->MergedFlags : payload_window->WindowClass.DockNodeFlagsOverrideSet;
    ImGuiDockNodeFlags dst_node_flags = host_node ? host_node->MergedFlags : host_window->WindowClass.DockNodeFlagsOverrideSet;
    data->IsCenterAvailable = true;
    if (is_outer_docking)
        data->IsCenterAvailable = false;
    else if (g.IO.ConfigDockingNoDockingOver)
        data->IsCenterAvailable = false;
    else if (dst_node_flags & ImGuiDockNodeFlags_NoDockingOverMe)
        data->IsCenterAvailable = false;
    else if (host_node && (dst_node_flags & ImGuiDockNodeFlags_NoDockingOverCentralNode) && host_node->IsCentralNode())
        data->IsCenterAvailable = false;
    else if ((!host_node || !host_node->IsEmpty()) && payload_node && payload_node->IsSplitNode() && (payload_node->OnlyNodeWithWindows == NULL)) // Is _visibly_ split?
        data->IsCenterAvailable = false;
    else if ((src_node_flags & ImGuiDockNodeFlags_NoDockingOverOther) && (!host_node || !host_node->IsEmpty()))
        data->IsCenterAvailable = false;
    else if ((src_node_flags & ImGuiDockNodeFlags_NoDockingOverEmpty) && host_node && host_node->IsEmpty())
        data->IsCenterAvailable = false;

    data->IsSidesAvailable = true;
    if ((dst_node_flags & ImGuiDockNodeFlags_NoDockingSplit) || g.IO.ConfigDockingNoSplit)
        data->IsSidesAvailable = false;
    else if (!is_outer_docking && host_node && host_node->ParentNode == NULL && host_node->IsCentralNode())
        data->IsSidesAvailable = false;
    else if (src_node_flags & ImGuiDockNodeFlags_NoDockingSplitOther)
        data->IsSidesAvailable = false;

    // Build a tentative future node (reuse same structure because it is practical. Shape will be readjusted when previewing a split)
    data->FutureNode.HasCloseButton = (host_node ? host_node->HasCloseButton : host_window->HasCloseButton) || (payload_window->HasCloseButton);
    data->FutureNode.HasWindowMenuButton = host_node ? true : ((host_window->Flags & ImGuiWindowFlags_NoCollapse) == 0);
    data->FutureNode.Pos = ref_node_for_rect ? ref_node_for_rect->Pos : host_window->Pos;
    data->FutureNode.Size = ref_node_for_rect ? ref_node_for_rect->Size : host_window->Size;

    // Calculate drop shapes geometry for allowed splitting directions
    IM_ASSERT(ImGuiDir_None == -1);
    data->SplitNode = host_node;
    data->SplitDir = ImGuiDir_None;
    data->IsSplitDirExplicit = false;
    if (!host_window->Collapsed)
        for (int dir = ImGuiDir_None; dir < ImGuiDir_COUNT; dir++)
        {
            if (dir == ImGuiDir_None && !data->IsCenterAvailable)
                continue;
            if (dir != ImGuiDir_None && !data->IsSidesAvailable)
                continue;