I was wondering if there is a way to enforce a straight pointer movement in macOS.

For instance when you are annotating a PDF document, you may want to make a straight stroke as an underline.

So what I'm looking for is a feature like when moving your pointers with the shift key pressed, then the pointer movement would be restricted to a straight line (either horizontal, vertical, or diagonal).

Is there such feature in macOS, or other applications which makes this possible?

1 Answer 1


You can, in fact, make specific dedicated underlines.

In the top bar, the pencil tool, there are options for Underline & strikethrough.
Both will track any text you select, letter, word or paragraph…

enter image description here

Giving this [the blue highlight is the selection, the red line is the added underline]

enter image description here

Instructions from my beer-making xmas prezzie ;)

  • Thanks for the reply! But I was looking for a way to restrict the mouse movement itself. The feature you suggested is provided from a lot of pdf viewer applications, but they kinda break when underlining math equations and so on.
    – Aesop
    Dec 29, 2020 at 4:46

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