You can open Google Inbox within its own tab. Just make sure you’re using Mailplane 3.6.0. If not, you can get it via ‘Mailplane 3 > Check for Updates…’.
Hint for early birds: The ‘Redirect Gmail to inbox.google.com’ workaround, promoted in the previous version of Mailplane, won’t work anymore. Gmail will always open inside the Gmail tab. Open a new tab if you’d like to use Google Inbox.
Unfortunately, trying to open an attachment will still cause a freeze. According to our investigations, it seems to be caused by an Google Inbox error in the background. We reported it to Google and hope to get a resolution anytime soon.
If you don’t mind, please report it via ‘Help & Feedback’ at the bottom of the Google Inbox sidebar as well. It might help to increase its priority.