Drag and Drop: Fix losing drop source ActiveID (and often source tooltip) when opening a TreeNode() or CollapsingHeader() while dragging. (#1738)

Amend 7b3d379, 8241cd62 etc.

docs/CHANGELOG.txt

@@ -57,6 +57,8 @@ Other Changes:
 - Tab Bar: Made it possible to append to an existing tab bar by calling BeginTabBar()/EndTabBar() again.
 - Tab Bar: Fixed using more than 128 tabs in a tab bar (scrolling policy recommended).
 - Tab Bar: Do not display a tooltip if the name already fits over a given tab. (#3521)
+- Drag and Drop: Fix losing drop source ActiveID (and often source tooltip) when opening a TreeNode()
+  or CollapsingHeader() while dragging. (#1738)
 - Drag and Drop: Fix drag and drop to tie same-size drop targets by choosen the later one. Fixes dragging
   into a full-window-sized dockspace inside a zero-padded window. (#3519, #2717) [@Black-Cat]
 - Backends: OpenGL3: Use glGetString(GL_VERSION) query instead of glGetIntegerv(GL_MAJOR_VERSION, ...) 

imgui.cpp

@@ -9425,6 +9425,8 @@ bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags)
             g.DragDropActive = true;
             g.DragDropSourceFlags = flags;
             g.DragDropMouseButton = mouse_button;
+            if (payload.SourceId == g.ActiveId)
+                g.ActiveIdNoClearOnFocusLoss = true;
         }
         g.DragDropSourceFrameCount = g.FrameCount;
         g.DragDropWithinSource = true;