Hi, i'm trying to get the parallel ruler to snap to horizontal/vertical. The guide says to shift click, but if I hold shift before (and while) clicking, nothing happens, the ruler doesn't even appear. If I press and hold shift after I click, it doesn't snap. What am I doing wrong?

The 'shift while drag' snapping only happens after the ruler has been created and is being adjusted using the handles at the end points.

Draw the ruler first by clicking on the canvas for the start point then dragging out a line and then clicking the end point. Then the ruler's small control box will appear with its move/visibility/delete icons. After this you can click-drag the end point handles. Press shift after the click or at any time during dragging to force the ruler to vertical or horizontal orientation.