The appeal here is somewhat complicated – as reflected by the Federal Circuit’s 42-page opinion.  The complications begin with the founding of EVE, and emulation software company founded by folks who invented emulation software at Mentor. Synopsys then acquired EVE.   EVE had previously licensed some of Mentor’s patents, but Mentor claims the license was terminated by the Synopsys acquisition.

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… The setup here is proper for en banc or Supreme Court petition.  On Point is Mark A. Lemley, Rethinking Assignor Estoppel, 54 Hous. L. Rev. 513 (2016) (arguing that the doctrine “interferes with both the invalidation of bad patents and the goal of employee mobility”).