In my Visual Studio 2012 Solution Explorer, when I single click a filename it opens it. This was different from Visual Studio 2010 (required a double click). Is there a way
The Single-click opens files in the preview tab text mentioned in rubber boots' answer is missing in VS 2015. Mysteriously, a search for "single click" still brings up the Environment > Tabs and Windows option.
So for VS2015 and VS2017, either click the button in Vinicius's answer, or uncheck the Preview selected files in Solution Explorer box shown in the screenshot below.