ImDrawList: Clarifying and guarateeing that CmdBuffer.back()->UserCallback should be always be NULL.

imgui_draw.cpp

@@ -451,7 +451,8 @@ void ImDrawList::_PopUnusedDrawCmd()
 void ImDrawList::AddCallback(ImDrawCallback callback, void* callback_data)
 {
     ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
-    if (curr_cmd->ElemCount != 0 || curr_cmd->UserCallback != NULL)
+    IM_ASSERT(curr_cmd->UserCallback == NULL);
+    if (curr_cmd->ElemCount != 0)
     {
         AddDrawCmd();
         curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
@@ -473,11 +474,12 @@ void ImDrawList::_OnChangedClipRect()
 {
     // If current command is used with different settings we need to add a new command
     ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
-    if ((curr_cmd->ElemCount != 0 && memcmp(&curr_cmd->ClipRect, &_CmdHeader.ClipRect, sizeof(ImVec4)) != 0) || curr_cmd->UserCallback != NULL)
+    if (curr_cmd->ElemCount != 0 && memcmp(&curr_cmd->ClipRect, &_CmdHeader.ClipRect, sizeof(ImVec4)) != 0)
     {
         AddDrawCmd();
         return;
     }
+    IM_ASSERT(curr_cmd->UserCallback == NULL);
 
     // Try to merge with previous command if it matches, else use current command
     ImDrawCmd* prev_cmd = curr_cmd - 1;
@@ -494,11 +496,12 @@ void ImDrawList::_OnChangedTextureID()
 {
     // If current command is used with different settings we need to add a new command
     ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
-    if ((curr_cmd->ElemCount != 0 && curr_cmd->TextureId != _CmdHeader.TextureId) || curr_cmd->UserCallback != NULL)
+    if (curr_cmd->ElemCount != 0 && curr_cmd->TextureId != _CmdHeader.TextureId)
     {
         AddDrawCmd();
         return;
     }
+    IM_ASSERT(curr_cmd->UserCallback == NULL);
 
     // Try to merge with previous command if it matches, else use current command
     ImDrawCmd* prev_cmd = curr_cmd - 1;
@@ -1427,6 +1430,11 @@ void ImDrawListSplitter::Merge(ImDrawList* draw_list)
         if (int sz = ch._IdxBuffer.Size) { memcpy(idx_write, ch._IdxBuffer.Data, sz * sizeof(ImDrawIdx)); idx_write += sz; }
     }
     draw_list->_IdxWritePtr = idx_write;
+
+    // Ensure there's always a non-callback draw command trailing the command-buffer
+    if (draw_list->CmdBuffer.Size == 0 || draw_list->CmdBuffer.back().UserCallback != NULL)
+        draw_list->AddDrawCmd();
+
     draw_list->_OnChangedClipRect(); // We call this instead of AddDrawCmd(), so that empty channels won't produce an extra draw call.
     draw_list->_OnChangedTextureID();
     _Count = 1;
@@ -1437,6 +1445,7 @@ void ImDrawListSplitter::SetCurrentChannel(ImDrawList* draw_list, int idx)
     IM_ASSERT(idx >= 0 && idx < _Count);
     if (_Current == idx)
         return;
+
     // Overwrite ImVector (12/16 bytes), four times. This is merely a silly optimization instead of doing .swap()
     memcpy(&_Channels.Data[_Current]._CmdBuffer, &draw_list->CmdBuffer, sizeof(draw_list->CmdBuffer));
     memcpy(&_Channels.Data[_Current]._IdxBuffer, &draw_list->IdxBuffer, sizeof(draw_list->IdxBuffer));