To achieve smooth transitions between animations in Godot using Spine, you should utilize the animation mixing feature provided by Spine. When you set a new animation to an AnimationState, it will be mixed with the previous animation according to the set mix duration, allowing for a smooth transition. You can set the mix duration using the setMix method on the AnimationStateData object. This will ensure that when you disable a SpineBoneNode or change animations, the transition is smooth rather than immediate.

For example, you can set the mix duration like this:

widget.state.data.setMix("previousAnimation", "newAnimation", 0.5);

This code sets a mix duration of 0.5 seconds between the "previousAnimation" and "newAnimation", allowing for a smooth transition.

Asta Spinebot's point of view is correct. Since SpineBoneNode itself can only be turned on or off, smooth transitions are not possible on its own. However, I believe that smooth transitions can be achieved by using "Mix".

One method is to use AnimationState mixing, as suggested by Spinebot. This assumes that the skeleton has an animation that sets the IK constraint mix to 100. (The aim animation that Spineboy has is exactly such an animation.)
In other words, by enabling the SpineBoneNode and mixing in the animation with a mix duration of 0.5 or so, you can set the arm to move smoothly toward the target.

The second method is to smoothly change the mix value of the IK constraint itself via code. By enabling the SpineBoneNode and incrementing the mix value every frame using a for loop, you can smoothly initiate following to the IK target bone. With spine-godot, you can find IK constraints using the find_ik_constraint() method and change the mix values using set_mix().