Drag and Drop: BeginDragDropSource() with ImGuiDragDropFlags_SourceAllowNullID doesn't lose tooltip when scrolling. (#143)

Reduced amount of self critical commentary since it'll appear like a hack for users but it isn't more a hack than many other things.

docs/CHANGELOG.txt

@@ -67,6 +67,8 @@ Other Changes:
   by the clipper to display. (#3841)
 - Clipper: fixed content height declaration slightly mismatching the value of when not using a clipper.
   (an additional ItemSpacing.y was declared, affecting scrollbar range).
+- Drag and Drop: BeginDragDropSource() with ImGuiDragDropFlags_SourceAllowNullID doesn't lose
+  tooltip when scrolling. (#143)
 - Backends: Vulkan: Call vkCmdSetScissor() at the end of render with a full-viewport to reduce
   likehood of issues with people using VK_DYNAMIC_STATE_SCISSOR in their app without calling
   vkCmdSetScissor() explicitly every frame. (#4644)

imgui.cpp

@@ -3121,7 +3121,7 @@ ImGuiID ImGuiWindow::GetIDNoKeepAlive(int n)
 ImGuiID ImGuiWindow::GetIDFromRectangle(const ImRect& r_abs)
 {
     ImGuiID seed = IDStack.back();
-    const int r_rel[4] = { (int)(r_abs.Min.x - Pos.x), (int)(r_abs.Min.y - Pos.y), (int)(r_abs.Max.x - Pos.x), (int)(r_abs.Max.y - Pos.y) };
+    ImRect r_rel = ImGui::WindowRectAbsToRel(this, r_abs);
     ImGuiID id = ImHashData(&r_rel, sizeof(r_rel), seed);
     ImGui::KeepAliveID(id);
     return id;
@@ -10276,16 +10276,16 @@ bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags)
                 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:
-            // A) Read the explanation below, B) Use the ImGuiDragDropFlags_SourceAllowNullID flag, C) Swallow your programmer pride.
+            // A) Read the explanation below, B) Use the ImGuiDragDropFlags_SourceAllowNullID flag.
             if (!(flags & ImGuiDragDropFlags_SourceAllowNullID))
             {
                 IM_ASSERT(0);
                 return false;
             }
 
-            // Magic fallback (=somehow reprehensible) to handle items with no assigned ID, e.g. Text(), Image()
+            // Magic fallback 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.
+            // THE IDENTIFIER WON'T SURVIVE ANY REPOSITIONING/RESIZINGG OF THE WIDGET, so if your widget moves your dragging operation will be canceled.
             // We don't need to maintain/call ClearActiveID() as releasing the button will early out this function and trigger !ActiveIdIsAlive.
             // Rely on keeping other window->LastItemXXX fields intact.
             source_id = g.LastItemData.ID = window->GetIDFromRectangle(g.LastItemData.Rect);