I get why you would hide interface elements to use the screen real estate for something else.
I have no idea why some interfaces hide elements hide and leave the space they'd taken up unused.
IntelliJ does this, for example, with the icons above the project tree. There is this little target disc that moves the selection in the project tree to the file currently open in the active editor tab. You have to know the secret spot on the screen where it is hidden and if you move your mouse pointer to the void there, it magically appears.
Why? What is the rationale behind going out of your way to implement something like this?
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