go off at a tangent

{UK} go / fly off at a tangent AND
{US} go off on a tangent (informal) — to suddenly start talking or thinking about a completely new subject or about something that doesn't concern the main subject of conversation; to suddenly change the line of thought or a course of action — 1. отклониться от темы; отвлечься от темы, уйти в сторону (от обсуждаемой темы) || 2. отклоняться от курса
Also: go off on tangents

Explanation: used with people who have a hard time staying on task

Example 1: It's hard to have a logical discussion with Ann because she never sticks to the point but keeps going off at a tangent/she's always going off at a tangent. — ... всегда уходит в сторону от (обсуждаемой) темы / отклоняется от темы/ начинает говорить не по теме, пр.

Example 2: Please stick to one subject and don't go off on a tangent. Don't go off on a tangent, stay on topic!

Example 3: If Ann would settle down and deal with one subject she would be all right, but she keeps going off on tangents

Example 4: She went off on a tangent during her presentation and lost her audience.

Related vocabulary
[jump the track]
beat about the bush
get off the point,
get off the subject,
side-track (to diverge from the main subject, course, or road)
get sidetracked

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(a reference to geometry)

Opp.:
stay on task
stick to the point
...