This feature has been introduced in Atom 1.6.0.
Basically if you open the file with a single click, you take a peek at the contents of a file without opening it in a dedicated tab. When another item is opened in a pane, it replaces the pending tab.
When you simply double-click the file or the tab, the tab is automagically made permanent for you.
See: Major Changes in 1.6.0 Stable - Pending Pane Items
Pending Pane Items
When you single-click a file from the tree view or the find-and-replace search results, the file in question opens as a pending pane item. When another item is opened in a pane, it replaces the pending tab in that pane instead of being appended to the list of tabs. This allows you to quickly click through several files quickly without opening each of them in a dedicated tab. In the default UI configuration, the tab’s title is italicized if the pane item is pending. If you want the tab to stick around permanently, simply double-click the file or the tab; in addition, when you edit text within a text editor that lives in a pending tab, the tab is automagically made permanent for you. You can disable this functionality by unchecking Allow Pending Pane Items in the Core Settings section of the editor settings.
