Docking: Fix honoring payload filter with overlapping nodes. (we incorrectly over-relied on g.HoveredDockNode when making change for #3398)

Essentially undo part of 85a661d (#3398) + ref cf31254 (#3420)

imgui.cpp

@@ -14890,18 +14890,26 @@ void ImGui::BeginDockableDragDropTarget(ImGuiWindow* window)
     if (AcceptDragDropPayload(IMGUI_PAYLOAD_TYPE_WINDOW, ImGuiDragDropFlags_AcceptBeforeDelivery | ImGuiDragDropFlags_AcceptNoDrawDefaultRect))
     {
         // Select target node
-        // (we should not assume that g.HoveredDockNode is != NULL when window is a host dock node: it depends on padding/spacing handled by DockNodeTreeFindVisibleNodeByPos)
-        ImGuiDockNode* node = g.HoveredDockNode;
-        const bool allow_null_target_node = window->DockNode == NULL && window->DockNodeAsHost == NULL;
+        // (Important: we cannot use g.HoveredDockNode here! Because each of our target node have filters based on payload, each candidate drop target will do its own evaluation)
+        bool dock_into_floating_window = false;
+        ImGuiDockNode* node = NULL;
+        if (window->DockNodeAsHost)
+        {
+            // Cannot assume that node will != NULL even though we passed the rectangle test: it depends on padding/spacing handled by DockNodeTreeFindVisibleNodeByPos().
+            node = DockNodeTreeFindVisibleNodeByPos(window->DockNodeAsHost, g.IO.MousePos);
 
-        // There is an edge case when docking into a dockspace which only has inactive nodes (because none of the windows are active)
+            // There is an edge case when docking into a dockspace which only has _inactive_ nodes (because none of the windows are active)
             // In this case we need to fallback into any leaf mode, possibly the central node.
-        if (window->DockNodeAsHost && node && node->IsDockSpace() && node->IsRootNode())
+            // FIXME-20181220: We should not have to test for IsLeafNode() here but we have another bug to fix first.
+            if (node && node->IsDockSpace() && node->IsRootNode())
+                node = (node->CentralNode && node->IsLeafNode()) ? node->CentralNode : DockNodeTreeFindFallbackLeafNode(node);
+        }
+        else
         {
-            if (node->CentralNode && node->IsLeafNode()) // FIXME-20181220: We should not have to test for IsLeafNode() here but we have another bug to fix first.
-                node = node->CentralNode;
+            if (window->DockNode)
+                node = window->DockNode;
             else
-                node = DockNodeTreeFindFallbackLeafNode(node);
+                dock_into_floating_window = true; // Dock into a regular window
         }
 
         const ImRect explicit_target_rect = (node && node->TabBar && !node->IsHiddenTabBar() && !node->IsNoTabBar()) ? node->TabBar->BarRect : ImRect(window->Pos, window->Pos + ImVec2(window->Size.x, GetFrameHeight()));
@@ -14910,7 +14918,7 @@ void ImGui::BeginDockableDragDropTarget(ImGuiWindow* window)
         // Preview docking request and find out split direction/ratio
         //const bool do_preview = true;     // Ignore testing for payload->IsPreview() which removes one frame of delay, but breaks overlapping drop targets within the same window.
         const bool do_preview = payload->IsPreview() || payload->IsDelivery();
-        if (do_preview && (node != NULL || allow_null_target_node))
+        if (do_preview && (node != NULL || dock_into_floating_window))
         {
             ImGuiDockPreviewData split_inner;
             ImGuiDockPreviewData split_outer;

imgui_internal.h

@@ -1296,7 +1296,7 @@ struct ImGuiContext
     ImGuiWindow*            HoveredWindow;                      // Window the mouse is hovering. Will typically catch mouse inputs.
     ImGuiWindow*            HoveredRootWindow;                  // == HoveredWindow ? HoveredWindow->RootWindow : NULL, merely a shortcut to avoid null test in some situation.
     ImGuiWindow*            HoveredWindowUnderMovingWindow;     // Hovered window ignoring MovingWindow. Only set if MovingWindow is set.
-    ImGuiDockNode*          HoveredDockNode;
+    ImGuiDockNode*          HoveredDockNode;                    // Hovered dock node.
     ImGuiWindow*            MovingWindow;                       // Track the window we clicked on (in order to preserve focus). The actual window that is moved is generally MovingWindow->RootWindow.
     ImGuiWindow*            WheelingWindow;                     // Track the window we started mouse-wheeling on. Until a timer elapse or mouse has moved, generally keep scrolling the same window even if during the course of scrolling the mouse ends up hovering a child window.
     ImVec2                  WheelingWindowRefMousePos;