How to set single clicking to open file in IntelliJ IDEA?

In the Project View click on the "gear" icon and enable the Open Files with Single Click option:

Open Files with Single Click

Use Ctrl+Q for Quick JavaDoc and Ctrl+Shift+I for the declaration pop-up.

Go to Preferences and turn on Enable preview tab, that will enable Open Files with Single Click for all projects.

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Now it's available in a simpler way. In the Project View click on the Gear icon and then click Open Files with Single Click:

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