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1. Use CTRL + ALT + S (or click on File > Settings) to open settings
2. Go to Editor > General - under the Virtual Space or Soft Wraps sub-group
3. Check "Use soft wraps in editor" or "Soft-wrap these files"
4. Add *; at text input like this:
[x] Soft-wrap these files: [ *.md; *.txt; *.rst; *.adoc; *; ]