Mortimer- Lancastrian War?

This may have come up in a slightly different version some time ago. Richard II becomes king when his grandfather dies in 1377. However, he dies suddenly in his fifteenth or sixteenth year, before attaining his majority, and obviously unmarried. The official heir is Roger Mortimer, his cousin, and still not of age at this time. His very ambitious uncle, John of Lancaster, either seizes the throne or claims the regency. Does a civil war break out between the Mortimers and Lancastrians now? Does a faction seek French support, but at the cost of returning Calais to France, and possibly even Gascony, including Bordeaux? Thoughts?
 
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