Mono in the early 2000s
Mono) — launched by Miguel de Icaza around 2001 — was the open-source implementation of Microsoft's.NET Common Language Infrastructure for non-Windows platforms. Running C# code on Mac OS X or Linux in 2002 was bleeding edge. Contributing to Mono in those years meant tracking the published ECMA spec while it was still moving, finding gaps where the spec was ambiguous, and shipping the first cross-platform corrections that would later become baseline behavior.
For Quark, the bet on Mono was strategic. QuarkXPress shipped on both Mac and Windows. Tooling that wanted to drive it from either side without forking the code paths needed a runtime that ran on both. Mono was the only credible answer in 2003. C# was already a more ergonomic choice than C++ for tooling code; Mono made that choice portable.