I still run an old copy of Peak BIAS on an old MBP running Snow Leopard because of this feature:
You can have a track, ,select a point on it, copy another track to your clipboard, then use “Mix” under the DSP menu, select a % to mix, then it overlays the new track so you can still hear some of the old track, depending on what % you selected. Then you can highlight the part of the original track near the insertion point and, again under the DSP menu, select “Adjust Gain Envelope” and you can then use a graphic tool to create a graduated transition from the original track to the mixed tracks. It’s hard to explain. Anyway it is quick and easy and I wish Audacity would integrate this feature.