Drag and Drop: BeginDragDropSource() with ImGuiDragDropFlags_SourceExtern assume a mouse button being pressed. (#143)

docs/CHANGELOG.txt

@@ -62,9 +62,11 @@ Other changes:
   Added corresponding ImGuiCol_TabSelectedOverline and ImGuiCol_TabDimmedSelectedOverline colors.
 - Drag and Drop: BeginDragDropSource() with ImGuiDragDropFlags_SourceExtern sets
   active id so a multi-frame extern source doesn't interfere with hovered widgets. (#143)
+- Drag and Drop: BeginDragDropSource() with ImGuiDragDropFlags_SourceExtern does not assume
+  a mouse button being pressed. Facilitate implementing cross-context drag and drop. (#143)
 - Drag and Drop: Fixes an issue when elapsing payload would be based on last payload
   frame instead of last drag source frame, which makes a difference if not resubmitting
-  payload every frame.
+  payload every frame. (#143)
 - Examples: GLFW+Vulkan, SDL+Vulkan: handle swap chain resize even without Vulkan
   returning VK_SUBOPTIMAL_KHR, which doesn't seem to happen on Wayland. (#7671)
   [@AndreiNego, @ocornut]

imgui.cpp

@@ -5136,7 +5136,7 @@ void ImGui::EndFrame()
     if (g.DragDropActive)
     {
         bool is_delivered = g.DragDropPayload.Delivery;
-        bool is_elapsed = (g.DragDropSourceFrameCount + 1 < g.FrameCount) && ((g.DragDropSourceFlags & ImGuiDragDropFlags_SourceAutoExpirePayload) || !IsMouseDown(g.DragDropMouseButton));
+        bool is_elapsed = (g.DragDropSourceFrameCount + 1 < g.FrameCount) && ((g.DragDropSourceFlags & ImGuiDragDropFlags_SourceAutoExpirePayload) || g.DragDropMouseButton == -1 || !IsMouseDown(g.DragDropMouseButton));
         if (is_delivered || is_elapsed)
             ClearDragDrop();
     }
@@ -13239,6 +13239,7 @@ bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags)
         window = NULL;
         source_id = ImHashStr("#SourceExtern");
         source_drag_active = true;
+        mouse_button = g.IO.MouseDown[0] ? 0 : -1;
         KeepAliveID(source_id);
         SetActiveID(source_id, NULL);
     }
@@ -13439,7 +13440,10 @@ const ImGuiPayload* ImGui::AcceptDragDropPayload(const char* type, ImGuiDragDrop
         RenderDragDropTargetRect(r, g.DragDropTargetClipRect);
 
     g.DragDropAcceptFrameCount = g.FrameCount;
-    payload.Delivery = was_accepted_previously && !IsMouseDown(g.DragDropMouseButton); // For extern drag sources affecting OS window focus, it's easier to just test !IsMouseDown() instead of IsMouseReleased()
+    if ((g.DragDropSourceFlags & ImGuiDragDropFlags_SourceExtern) && g.DragDropMouseButton == -1)
+        payload.Delivery = was_accepted_previously && (g.DragDropSourceFrameCount < g.FrameCount);
+    else
+        payload.Delivery = was_accepted_previously && !IsMouseDown(g.DragDropMouseButton); // For extern drag sources affecting OS window focus, it's easier to just test !IsMouseDown() instead of IsMouseReleased()
     if (!payload.Delivery && !(flags & ImGuiDragDropFlags_AcceptBeforeDelivery))
         return NULL;