Anchor point adjusting with a constrain
When adjusting the length of a path by dragging an anchor point that's set at ANY angle, not just 90 or 45, hold shift + ctrl (or something) to keep it locked at whatever angle it's already at, instead of it going all over the place. Just keep the angle locked and be able to lengthen or shorten the path with ease!!
Many things to say about it.
1. If you enable Smart Guides and disable Snap to Grid and drag an end sharp point accurately enough (yeah, boring), illustrator snaps it to an angled virtual Line Extension guide, that does exactly what is wanted... but it won’t work for a middle point, or a point that has a handle, or a point that is adjacent to a point with a handle... so many ifs!
2. There is a commercial Line Extend plugin from AstuteGraphics pack, that does extending (not moving though) flawlessly, with incrementations, highlighting, other options. Also, their PathScribe tool allows linear constrain for moving a point.
3. Sure it should be in Illustrator from the box!
Stéphanie Noverraz commented
It would be nice if we could lengthen or shorten a path (say, along selected anchor point's tangent) by a set distance (or even >to< a set distance or size).
A bit like the Scale tool, but with absolute values instead of %.