Tool window bars empty in IntelliJ IDE

Problem

The tool windows in IntelliJ IDEA unexpectedly disappeared, while the tool window bars were still present. What could have happened? How to revert?

Resolution

Reason 1: Zen mode

To enter Zen mode, use "View | Appearance | Enter Zen mode" or assign a shortcut.

If you accidently entered Zen mode or Distraction free mode, you tool windows will disappear. To bring them back, exit with "View | Appearance | Exit Zen mode"

Reason 2: empty windows layout was restored

User can save/load a tool window layout.

It is possible that a predefined key combination accidentally triggered a layout change or hid the tool windows. To resolve this:

1. Re-display any missing tool windows by selecting them from the View -> Tool Windows menu.
2. Save your current layout once restored and configured to your liking under Window -> Store Current Layout as Default.
3. To avoid this in the future, familiarize yourself with the key combinations under Settings -> Keymap. Additionally, you can disable or modify shortcuts to prevent accidental triggers.