ImDrawList: Fix broken channel splitting (broken by d1e8b69) (#2591)

imgui_draw.cpp

@@ -568,18 +568,27 @@ void ImDrawList::ChannelsMerge()
     if (CmdBuffer.Size && CmdBuffer.back().ElemCount == 0)
         CmdBuffer.pop_back();
 
-    int new_cmd_buffer_count = 0, new_idx_buffer_count = 0;
+    // Calculate our final buffer sizes. Also fix the incorrect IdxOffset values in each command.
+    int new_cmd_buffer_count = 0;
+    int new_idx_buffer_count = 0;
+    int idx_offset = CmdBuffer.back().IdxOffset + CmdBuffer.back().ElemCount;
     for (int i = 1; i < _ChannelsCount; i++)
     {
         ImDrawChannel& ch = _Channels[i];
         if (ch.CmdBuffer.Size && ch.CmdBuffer.back().ElemCount == 0)
             ch.CmdBuffer.pop_back();
         new_cmd_buffer_count += ch.CmdBuffer.Size;
         new_idx_buffer_count += ch.IdxBuffer.Size;
+        for (int cmd_n = 0; cmd_n < ch.CmdBuffer.Size; cmd_n++)
+        {
+            ch.CmdBuffer.Data[cmd_n].IdxOffset = idx_offset;
+            idx_offset += ch.CmdBuffer.Data[cmd_n].ElemCount;
+        }
     }
     CmdBuffer.resize(CmdBuffer.Size + new_cmd_buffer_count);
     IdxBuffer.resize(IdxBuffer.Size + new_idx_buffer_count);
 
+    // Flatten our N channels at the end of the first one.
     ImDrawCmd* cmd_write = CmdBuffer.Data + CmdBuffer.Size - new_cmd_buffer_count;
     _IdxWritePtr = IdxBuffer.Data + IdxBuffer.Size - new_idx_buffer_count;
     for (int i = 1; i < _ChannelsCount; i++)