Pinning and Unpinning Tabs
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IntelliJ IDEA can limit the number of tabs opened in the editor simultaneously. When the number of tabs reaches its limit, the editor closes the tabs according to the tab closing policy, that is defined in the Editor Tabs settings page. By default, the tab limit is 10, but you can change it if necessary.
To prevent a tab from being closed automatically, you can pin this tab. Besides, when you close the editor tabs, you have an option to preserve pinned tabs opened and close only the unpinned tabs.
When a tab is pinned, there is a special marker on it.