Leave a note on any file, and keep it on top while you work.
Right-click a file in Windows Explorer and choose Comment..., or select it and press Ctrl+Win+Alt+C. If you're working in another application with a file open (via Window+), the same shortcut comments on that open file instead of whatever's selected in Explorer.
The comment window includes Bold, Italic, Underline, Strikethrough, Checklist, and Link formatting, plus a pin button to keep the note always on top while you work.
Use the overflow menu to pick a color theme, clear the note or delete it entirely, show the file in Explorer, or open Settings. You can also set the window's opacity separately for when it's focused versus unfocused.
Comments are saved directly into the file's Comments property (or alongside it, for file types that don't support embedded properties), so they travel with the file and show up in Explorer's Comments column and search — no separate comment database to keep in sync.
The Comment context menu item and hotkey can each be turned off independently in the Settings app if you don't use them.