This was discussed in the Animation & Rigging module meeting, and the module members approved of this idea.

@bassam kurdali (bassamk) said: Check what happens if both handles and keys themselves are selected. That should move the key and keep the handles as they are. This is not what 3D Curve objects do -- they snap handles and points themselves.

In D13606#364220, @Sybren A. Stüvel (sybren) wrote:

This was discussed in the Animation & Rigging module meeting, and the module members approved of this idea.

@bassam kurdali (bassamk) said: Check what happens if both handles and keys themselves are selected. That should move the key and keep the handles as they are. This is not what 3D Curve objects do -- they snap handles and points themselves.

OKi, will complete it then

In D13606#364220, @Sybren A. Stüvel (sybren) wrote:

@bassam kurdali (bassamk) said: Check what happens if both handles and keys themselves are selected. That should move the key and keep the handles as they are. This is not what 3D Curve objects do -- they snap handles and points themselves.

After looking at it again, it is not entirely clear what "move the key and keep the handles as they are" really means if both handles and the keyframe itself are selected.
In all of the following, I also assume we should only really care about FREE and ALIGNED handles (since for the other modes handles would update in the background anyways).
Also: what happens if left handle and keyframe are selected (but not the right handle).
The transform system will keep the offset from the keyframe to the handle as soon as the keyframe is selected.
Of course we could just do the same for snapping (would be consistent).
However we loose the flexibility of keeping handles exactly in place and only snap the keyframe itself (not being an animator, I am a bit unsure how useful this really is though).

So for maximum flexibility, I'll update the patch with the following behavior:

• only keyframe is selected: just the keyframe moves in time (both handles stay exactly [absolute] where they are)
• only handle is selected: just the handle gets snapped, the keyframe stays exactly [absolute] where it is
• one handle plus keyframe selected: the keyframe gets snapped, the selected handle keeps it relative offset, the unselected handle stays exactly where it is
• both handles plus keyframe selected : keyframe gets snapped, both handles keep their relative distance (just is most like the transform system

(atm - current master - only the keyframe snaps, the handles always stay exactly where they are)

@bassam kurdali (bassamk) , @Kevin C. Burke (blastframe) , @Sybren A. Stüvel (sybren) : is this how you would expect it to work?

One issue I just noticed: if you box select multiple keys of multiple fcurves (all handles and keyframes are then selected), then deselect the keyframes (to work on multiple handles only), snapping will only work on one curve.
(this might have to do with the active FCurve, will check shortly...)