No.
Windows
1.0.0-20839f9b
Bug?
No.
The maximize button is there, but greyed out.
I have two Gmail accounts connected to the client atm, if that helps.

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Feature Request?
Not sure if this is just some sort of mistake or on purpose, but I would appreciate being able to maximize my window.
Wow this is a pretty strange one... thanks for reporting this. Looks like it's broken for me as well. I'll see if I can get this fixed in the next release.
Hey! This turned out to be a holdover from when Nylas Mail had a "Loading..." screen - it's been fixed in the 1.0.2 release that went live a few minutes ago. Enjoy!
Most helpful comment
Wow this is a pretty strange one... thanks for reporting this. Looks like it's broken for me as well. I'll see if I can get this fixed in the next release.