Most highly-capable video editors cannot edit and merge AVCHD clips without re-rendering everything, from which you lose quality. PowerDirector 11 claims to "smart render" - not recompress if not necessary - but it plain does not work with all clips in my own experience.
The free included software with Panasonic camcorders and Sony (HDWriter and PlayMemories) do not recompress unecessarily. But:
1. They only work with files from the one camera brand - e.g., you cannot use Playmemories to edit Panasonic AVCHD clip even though they appear to have the same specs.
2. They do not have title or transition effects capabilities.
3. They do not have frame-level editing capability.
4. PlayMemories does not create completely glitch free clip transitions in audio.
TMPGEnc MPEG Smart Renderer 4 does not have any of the shortcomings 1-4. It smart renders, has transition effects and titling, works with Sony and Panasonic clips, allows frame-level cuts, and creates absolutely glitch-free transitions.
I had no software that would smart render and merge Panasonic LX7 clips, so I looked around. This software did the trick. And this video was created using it (the LX7 museuem video):
You will note how the clips flow seamlessly, audio and video. I did not use any transition tricks (I generally hate such things), but you will see a fade at the end - that could not be done with PlayMemories or HDWriter (even if they could edit the files, which they could not).
I got this tip from Shield (Shawn), who posts here and on DPReview.