ImDrawList: Additional comments and extracted bits into ImDrawList::PopUnusedDrawCmd()

docs/CHANGELOG.txt

@@ -61,6 +61,8 @@ Other Changes:
   [@thedmd, @Shironekoben, @sergeyn, @ocornut]
 - ImDrawList, ImDrawListSplitter, Columns: Fixed an issue where starting a split when current 
   VtxOffset was not zero would lead to draw commands with wrong VtxOffset. (#2591)
+- ImDrawList, ImDrawListSplitter, Columns: Fixed an issue where starting a split right after
+  a callback draw command would incorrectly override the callback draw command.
 - Misc, Freetype: Fix for rare case where FT_Get_Char_Index() succeed but FT_Load_Glyph() fails.
 - CI: Added CI test to verify we're never accidentally dragging libstdc++ (on some compiler setups,
   static constructors for non-pod data seems to drag in libstdc++ due to thread-safety concerns).

imgui.cpp

@@ -4077,18 +4077,12 @@ static void AddWindowToSortBuffer(ImVector<ImGuiWindow*>* out_sorted_windows, Im
 
 static void AddDrawListToDrawData(ImVector<ImDrawList*>* out_list, ImDrawList* draw_list)
 {
+    // Remove trailing command if unused.
+    // Technically we could return directly instead of popping, but this make things looks neat in Metrics window as well.
+    draw_list->PopUnusedDrawCmd();
     if (draw_list->CmdBuffer.Size == 0)
         return;
 
-    // Remove trailing command if unused
-    ImDrawCmd* curr_cmd = &draw_list->CmdBuffer.back();
-    if (curr_cmd->ElemCount == 0 && curr_cmd->UserCallback == NULL)
-    {
-        draw_list->CmdBuffer.pop_back();
-        if (draw_list->CmdBuffer.Size == 0)
-            return;
-    }
-
     // Draw list sanity check. Detect mismatch between PrimReserve() calls and incrementing _VtxCurrentIdx, _VtxWritePtr etc.
     // May trigger for you if you are using PrimXXX functions incorrectly.
     IM_ASSERT(draw_list->VtxBuffer.Size == 0 || draw_list->_VtxWritePtr == draw_list->VtxBuffer.Data + draw_list->VtxBuffer.Size);

imgui.h

@@ -2061,6 +2061,7 @@ struct ImDrawList
     // NB: all primitives needs to be reserved via PrimReserve() beforehand!
     IMGUI_API void  ResetForNewFrame();
     IMGUI_API void  ClearFreeMemory();
+    IMGUI_API void  PopUnusedDrawCmd();
     IMGUI_API void  PrimReserve(int idx_count, int vtx_count);
     IMGUI_API void  PrimUnreserve(int idx_count, int vtx_count);
     IMGUI_API void  PrimRect(const ImVec2& a, const ImVec2& b, ImU32 col);      // Axis aligned rectangle (composed of two triangles)

imgui_draw.cpp

@@ -437,6 +437,17 @@ void ImDrawList::AddDrawCmd()
     CmdBuffer.push_back(draw_cmd);
 }
 
+// Pop trailing draw command (used before merging or presenting to user)
+// Note that this leaves the ImDrawList in a state unfit for further commands, as most code assume that CmdBuffer.Size > 0 && CmdBuffer.back().UserCallback == NULL
+void ImDrawList::PopUnusedDrawCmd()
+{
+    if (CmdBuffer.Size == 0)
+        return;
+    ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
+    if (curr_cmd->ElemCount == 0 && curr_cmd->UserCallback == NULL)
+        CmdBuffer.pop_back();
+}
+
 void ImDrawList::AddCallback(ImDrawCallback callback, void* callback_data)
 {
     ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
@@ -1362,13 +1373,12 @@ void ImDrawListSplitter::Split(ImDrawList* draw_list, int channels_count)
 
 void ImDrawListSplitter::Merge(ImDrawList* draw_list)
 {
-    // Note that we never use or rely on channels.Size because it is merely a buffer that we never shrink back to 0 to keep all sub-buffers ready for use.
+    // Note that we never use or rely on _Channels.Size because it is merely a buffer that we never shrink back to 0 to keep all sub-buffers ready for use.
     if (_Count <= 1)
         return;
 
     SetCurrentChannel(draw_list, 0);
-    if (draw_list->CmdBuffer.Size != 0 && draw_list->CmdBuffer.back().ElemCount == 0)
+    draw_list->PopUnusedDrawCmd();
-        draw_list->CmdBuffer.pop_back();
 
     // Calculate our final buffer sizes. Also fix the incorrect IdxOffset values in each command.
     int new_cmd_buffer_count = 0;
@@ -1378,8 +1388,11 @@ void ImDrawListSplitter::Merge(ImDrawList* draw_list)
     for (int i = 1; i < _Count; i++)
     {
         ImDrawChannel& ch = _Channels[i];
+
+        // Equivalent of PopUnusedDrawCmd() for this channel's cmdbuffer and except we don't need to test for UserCallback.
         if (ch._CmdBuffer.Size > 0 && ch._CmdBuffer.back().ElemCount == 0)
             ch._CmdBuffer.pop_back();
+
         if (ch._CmdBuffer.Size > 0 && last_cmd != NULL)
         {
             ImDrawCmd* next_cmd = &ch._CmdBuffer[0];