Help: merging two ImDrawLists

[actondev]

Not an issue, but not also a build/link/run issue: I hope I'm not abusing this place.

I asked this once in #3515 (comment) , but asking here again.
What are the steps needed to merge 2 ImDrawLists?

What I wanted to achieve is

• replace the current window drawlist

auto cacheData = new ImDrawListSharedData();
auto cacheDrawList = new ImDrawList(cacheData);
auto originalDrawList = ImGui::GetCurrentWindow()->DrawList;
ImGui::GetCurrentWindow()->DrawList = cacheDrawList;

• call other functions that draw things
• restore original draw list
• (how?) merge the cacheDrawList to the original draw list

I reached to the following

for (int i = 0; i < cacheDrawList->CmdBuffer.Size; i++) {
originalDrawList->CmdBuffer.push_back(cmd);
}
originalDrawList->PrimReserve(cacheDrawList->IdxBuffer.Size, cacheDrawList->VtxBuffer.Size);

for (int i = 0; i < cacheDrawList->VtxBuffer.Size; i++) {
  const ImDrawVert vert = cacheDrawList->VtxBuffer[i];
  originalDrawList->PrimWriteVtx(vert.pos, vert.uv, vert.col);
}
for (int i = 0; i < cacheDrawList->IdxBuffer.Size; i++) {
  originalDrawList->PrimWriteIdx(cacheDrawList->IdxBuffer[i]);
}

but sometimes it crashes, and when not, I don't see what was expected to be drawn. Also, later calls (using the normal/original draw list) do not get shown in the screen.

From what I saw in ImDrawListSplitter::Merge, I guess it has something to do with the cmd->IdxOffset.
But I really don't know much (or anything) about opengl/generally graphics representation, so cannot really follow the logic there.