A tool I discovered today - Keep a chosen application (presumably, your todo application) visible on the right of your primary screen at all times. Sadly it's only for mac. https://github.com/rxhanson/Rectangle/wiki/Todo-Mode

For Windows 10, just install Windows PowerToys. The Fancy Zones window docking utility is included with it.

Nice! I used Spectacle till now. This has some extra features like snapping to the edges/corners

For Linux users on KDE Plasma, you can just right click the title bar of any window, go to "More Actions" submenu and click "Show on all workspaces" + "Always keep on top". This comes pretty handy during demos/presentations.