Internals: Added BeginDragDropTooltip() internal function to convey semantic (drag and drop tooltip doesn't get clipped within display boundaries). Revert part of 3218666fb9. (#1739, #143).

TODO.txt

@@ -163,7 +163,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - tooltip: drag and drop with tooltip near monitor edges lose/changes its last direction instead of locking one. The drag and drop tooltip should always follow without changing direction.
  - tooltip: tooltip that doesn't fit in entire screen seems to lose their "last preferred direction" and may teleport when moving mouse.
  - tooltip: allow to set the width of a tooltip to allow TextWrapped() etc. while keeping the height automatic.
- - tooltip: allow tooltips with timers? or general timer policy? (instantaneous vs timed)
+ - tooltip: tooltips with delay timers? or general timer policy? (instantaneous vs timed) (#1485)
  
  - menus: calling BeginMenu() twice with a same name doesn't append as Begin() does for regular windows (#1207)
  - menus: menu bars inside modals windows are acting weird.
@@ -215,7 +215,6 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - filters: handle wildcards (with implicit leading/trailing *), regexps
  - filters: fuzzy matches (may use code at blog.forrestthewoods.com/4cffeed33fdb)
 
- - drag and drop: allow drag target to alter tooltip.
  - drag and drop: add demo. (#143, #479)
  - drag and drop: have some way to know when a drag begin from BeginDragDropSource() pov
  - drag and drop: allow using with other mouse buttons (where activeid won't be set). (#1637)

imgui.cpp

@@ -13230,11 +13230,9 @@ bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags)
 
         if (!(flags & ImGuiDragDropFlags_SourceNoPreviewTooltip))
         {
-            SetNextWindowBgAlpha(g.Style.Colors[ImGuiCol_PopupBg].w * 0.60f);
-            BeginTooltipEx(0, true);
-
             // Target can request the Source to not display its tooltip (we use a dedicated flag to make this request explicit)
             // We unfortunately can't just modify the source flags and skip the call to BeginTooltip, as caller may be emitting contents. 
+            BeginDragDropTooltip();
             if (g.DragDropActive && g.DragDropAcceptIdPrev && (g.DragDropAcceptFlags & ImGuiDragDropFlags_AcceptNoPreviewTooltip))
             {
                 ImGuiWindow* tooltip_window = g.CurrentWindow;
@@ -13257,13 +13255,32 @@ void ImGui::EndDragDropSource()
     IM_ASSERT(g.DragDropActive);
 
     if (!(g.DragDropSourceFlags & ImGuiDragDropFlags_SourceNoPreviewTooltip))
-        EndTooltip();
+        EndDragDropTooltip();
 
     // Discard the drag if have not called SetDragDropPayload()
     if (g.DragDropPayload.DataFrameCount == -1)
         ClearDragDrop();
 }
 
+void ImGui::BeginDragDropTooltip()
+{
+    // The default tooltip position is a little offset to give space to see the context menu (it's also clamped within the current viewport/monitor)
+    // In the context of a dragging tooltip we try to reduce that offset and we enforce following the cursor.
+    // Whatever we do we want to call SetNextWindowPos() to enforce a tooltip position and disable clipping the tooltip without our display area, like regular tooltip do.
+    ImGuiContext& g = *GImGui;
+    //ImVec2 tooltip_pos = g.IO.MousePos - g.ActiveIdClickOffset - g.Style.WindowPadding;
+    ImVec2 tooltip_pos = g.IO.MousePos + ImVec2(16 * g.Style.MouseCursorScale, 8 * g.Style.MouseCursorScale);
+    SetNextWindowPos(tooltip_pos);
+    SetNextWindowBgAlpha(g.Style.Colors[ImGuiCol_PopupBg].w * 0.60f);
+    //PushStyleVar(ImGuiStyleVar_Alpha, g.Style.Alpha * 0.60f); // This would be nice but e.g ColorButton with checkboard has issue with transparent colors :(
+    BeginTooltipEx(0, true);
+}
+
+void ImGui::EndDragDropTooltip()
+{
+    EndTooltip();
+}
+
 // Use 'cond' to choose to submit payload on drag start or every frame
 bool ImGui::SetDragDropPayload(const char* type, const void* data, size_t data_size, ImGuiCond cond)
 {

imgui_internal.h

@@ -1111,6 +1111,8 @@ namespace ImGui
     IMGUI_API bool          BeginDragDropTargetCustom(const ImRect& bb, ImGuiID id);
     IMGUI_API void          ClearDragDrop();
     IMGUI_API bool          IsDragDropPayloadBeingAccepted();
+    IMGUI_API void          BeginDragDropTooltip();
+    IMGUI_API void          EndDragDropTooltip();
 
     // FIXME-WIP: New Columns API
     IMGUI_API void          BeginColumns(const char* str_id, int count, ImGuiColumnsFlags flags = 0); // setup number of columns. use an identifier to distinguish multiple column sets. close with EndColumns().