Fix D3D12 Debug Layer warning if scissor rect is 0 width or 0 height

[StoneWolf]

In the event where the scissor rect is 0 width or 0 height, don't call Draw, as it generates warnings if the D3D12 Debug Layer is enabled, and nothing would have been drawn anyway.

Here's the warning that was being emitted:
D3D12 WARNING: ID3D12CommandList::DrawIndexedInstanced: Viewport: 0 is non-empty while the corresponding scissor rectangle is empty. Nothing will be written to the render target when this viewport is selected. In D3D12, scissor testing is always enabled. [ EXECUTION WARNING #695: DRAW_EMPTY_SCISSOR_RECTANGLE]

[ocornut]

Thank you for this! Linking to #3462, this has been reported recently. When I looked at it the other day I couldn’t repro. Do you have a clear repro to provide? The reason is that I would like to investigate if perhaps this should/could be fixed in the core instead of being fixed in the backend.

[StoneWolf]

I managed to create a simple repro. I should add that the bug happens on all three versions I tested (v1.78, latest zip from master and latest zip from the docking branch).

Easiest way to reproduce the bug is by using your example_win32_directx12 project:
In main.cpp, add after ImGui::NewFrame():

if (ImGui::Begin("Repro"))
{
    ImGui::BeginChild("Repro", ImVec2(150, 0), true);
    ImGui::EndChild();
}
ImGui::End();

Then drag the window all the way down and you should see the warnings appear. BeginChild/EndChild seem to be the culprits, since commenting them makes the warning go away.

[ocornut]

Thank you @StoneWolf this is now merged!