Docking: Fix window menu button. Broken by 3f16a52 (#4043)

Worked on single-frame click.

imgui.cpp

@@ -11638,6 +11638,7 @@ static void ImGui::UpdateViewportsNewFrame()
     // - when releasing a moving window we will revert to aiming behind (at viewport_hovered)
     // - when we are between viewports, our dragged preview will tend to show in the last viewport _even_ if we don't have tooltips in their viewports (when lacking monitor info)
     // - consider the case of holding on a menu item to browse child menus: even thou a mouse button is held, there's no active id because menu items only react on mouse release.
+    // FIXME-VIEWPORT: This is essentially broken, when ImGuiBackendFlags_HasMouseHoveredViewport is set we want to trust when viewport_hovered==NULL and use that.
     const bool is_mouse_dragging_with_an_expected_destination = g.DragDropActive;
     if (is_mouse_dragging_with_an_expected_destination && viewport_hovered == NULL)
         viewport_hovered = g.MouseLastHoveredViewport;
@@ -12970,7 +12971,6 @@ bool ImGui::DockContextCalcDropPosForDocking(ImGuiWindow* target, ImGuiDockNode*
 ImGuiDockNode::ImGuiDockNode(ImGuiID id)
 {
     ID = id;
-    WindowMenuButtonId = ImHashStr("#COLLAPSE", 0, ID);
     SharedFlags = LocalFlags = ImGuiDockNodeFlags_None;
     ParentNode = ChildNodes[0] = ChildNodes[1] = NULL;
     TabBar = NULL;
@@ -13823,8 +13823,7 @@ static void ImGui::DockNodeUpdateTabBar(ImGuiDockNode* node, ImGuiWindow* host_w
     // Docking/Collapse button
     if (has_window_menu_button)
     {
-        IMGUI_TEST_ENGINE_ID_INFO(node->WindowMenuButtonId, ImGuiDataType_String, "#COLLAPSE");
-        if (CollapseButton(node->WindowMenuButtonId, window_menu_button_pos, node))
+        if (CollapseButton(host_window->GetID("#COLLAPSE"), window_menu_button_pos, node)) // == DockNodeGetWindowMenuButtonId(node)
             OpenPopup("#WindowMenu");
         if (IsItemActive())
             focus_tab_id = tab_bar->SelectedTabId;

imgui_internal.h

@@ -1227,7 +1227,6 @@ enum ImGuiDockNodeState
 struct IMGUI_API ImGuiDockNode
 {
     ImGuiID                 ID;
-    ImGuiID                 WindowMenuButtonId;         // == ImHashStr("#COLLAPSE", ID)
     ImGuiDockNodeFlags      SharedFlags;                // Flags shared by all nodes of a same dockspace hierarchy (inherited from the root node)
     ImGuiDockNodeFlags      LocalFlags;                 // Flags specific to this node
     ImGuiDockNodeState      State;
@@ -2594,6 +2593,7 @@ namespace ImGui
     IMGUI_API void          DockNodeEndAmendTabBar();
     inline ImGuiDockNode*   DockNodeGetRootNode(ImGuiDockNode* node)                 { while (node->ParentNode) node = node->ParentNode; return node; }
     inline int              DockNodeGetDepth(const ImGuiDockNode* node)              { int depth = 0; while (node->ParentNode) { node = node->ParentNode; depth++; } return depth; }
+    inline ImGuiID          DockNodeGetWindowMenuButtonId(const ImGuiDockNode* node) { return ImHashStr("#COLLAPSE", 0, node->ID); }
     inline ImGuiDockNode*   GetWindowDockNode()                                      { ImGuiContext& g = *GImGui; return g.CurrentWindow->DockNode; }
     IMGUI_API bool          GetWindowAlwaysWantOwnTabBar(ImGuiWindow* window);
     IMGUI_API void          BeginDocked(ImGuiWindow* window, bool* p_open);