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Core parts of VS's UI (editor, shell) were rewritten in WPF in VS2010, and it's been the default framework for anything newly added or overhauled since then. So
by contextfree 10y ago
Core parts of VS's UI (editor, shell) were rewritten in WPF in VS2010, and it's been the default framework for anything newly added or overhauled since then. Some parts of VS still use other UI frameworks though, which makes it even harder to port. (and as you say the UI stuff is just one aspect of VS's very broad Windows API usage).