Drag and Drop: Fixed using BeginDragDropSource() within a Begin()/BeginChild() that returned false. (#4515) + BeginDragDropTarget()

Note how 79ae6d3b adedd a SkipItems test in BeginDragDropTargetCustom() only.
Catching this similar to work needed to neatly represent the error in #4375 #4158, #4008, #2562

docs/CHANGELOG.txt

@@ -52,6 +52,7 @@ Other Changes:
 - Nav: Fixed toggling menu layer with Alt or exiting menu layer with Esc not moving mouse when
   the NavEnableSetMousePos config flag is set.
 - Menus: adjust closing logic to accomodate for varying font size and dpi.
+- Drag and Drop: Fixed using BeginDragDropSource() inside a BeginChild() that returned false. (#4515)
 - PlotHistogram: Fixed zero-line position when manually specifying min<0 and max>0. (#4349) [@filippocrocchini]
 - IO: Added 'io.WantCaptureMouseUnlessPopupClose' alternative to `io.WantCaptureMouse'. (#4480)
   This allows apps to receive the click on void when that click is used to close popup (by default,

imgui.cpp

@@ -7813,6 +7813,7 @@ ImVec2 ImGui::GetWindowContentRegionMax()
 
 // Lock horizontal starting position + capture group bounding box into one "item" (so you can use IsItemHovered() or layout primitives such as SameLine() on whole group, etc.)
 // Groups are currently a mishmash of functionalities which should perhaps be clarified and separated.
+// FIXME-OPT: Could we safely early out on ->SkipItems?
 void ImGui::BeginGroup()
 {
     ImGuiContext& g = *GImGui;
@@ -9978,14 +9979,16 @@ bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags)
                 return false;
             if (g.ActiveIdMouseButton != -1)
                 mouse_button = g.ActiveIdMouseButton;
-            if (g.IO.MouseDown[mouse_button] == false)
+            if (g.IO.MouseDown[mouse_button] == false || window->SkipItems)
                 return false;
             g.ActiveIdAllowOverlap = false;
         }
         else
         {
             // Uncommon path: items without ID
-            if (g.IO.MouseDown[mouse_button] == false)
+            if (g.IO.MouseDown[mouse_button] == false || window->SkipItems)
+                return false;
+            if ((g.LastItemData.StatusFlags & ImGuiItemStatusFlags_HoveredRect) == 0 && (g.ActiveId == 0 || g.ActiveIdWindow != window))
                 return false;
 
             // If you want to use BeginDragDropSource() on an item with no unique identifier for interaction, such as Text() or Image(), you need to:
@@ -9996,10 +9999,6 @@ bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags)
                 return false;
             }
 
-            // Early out
-            if ((g.LastItemData.StatusFlags & ImGuiItemStatusFlags_HoveredRect) == 0 && (g.ActiveId == 0 || g.ActiveIdWindow != window))
-                return false;
-
             // Magic fallback (=somehow reprehensible) to handle items with no assigned ID, e.g. Text(), Image()
             // We build a throwaway ID based on current ID stack + relative AABB of items in window.
             // THE IDENTIFIER WON'T SURVIVE ANY REPOSITIONING OF THE WIDGET, so if your widget moves your dragging operation will be canceled.
@@ -10165,7 +10164,7 @@ bool ImGui::BeginDragDropTarget()
     if (!(g.LastItemData.StatusFlags & ImGuiItemStatusFlags_HoveredRect))
         return false;
     ImGuiWindow* hovered_window = g.HoveredWindowUnderMovingWindow;
-    if (hovered_window == NULL || window->RootWindow != hovered_window->RootWindow)
+    if (hovered_window == NULL || window->RootWindow != hovered_window->RootWindow || window->SkipItems)
         return false;
 
     const ImRect& display_rect = (g.LastItemData.StatusFlags & ImGuiItemStatusFlags_HasDisplayRect) ? g.LastItemData.DisplayRect : g.LastItemData.Rect;