By holding down SHIFT while clicking Compose an email or click to open an email from your inbox or expand an already open email, the email will be opened in a new window.
It is always useful to open an email in a new window.
SHIFT + CLICK allows you to do just that. Here are the SHIFT CLICKS that open a new windows:
SHIFT + Compose Click to compose an email in a new window
SHIFT + CLICK on an email in your inbox to open the email in a new window
SHIFT + CLICK on the expand arrow icon in the top right of an open email
Let us know what you think about this tip or if you find any more places you can use the SHIFT button to open a new tab in Gmail, in the Comments below.