Quicktime Player doesn't edit

MPEG Streamclip (very good, works up to 32-bit Mojave -- there with some cosmetic issues, free as a beer), QuickTime Player Pro 7 (quite good, no longer sold), current QuickTime Player (good, less options), Avidemux (quite good, many options that might puzzle at first, free as a beer) to losslessly trim and manipulate movies.

In addition to the good suggestions made earlier, there are a whole bunch of free higher-end editing tools. A lot of learning curve though. I use StreamClip on my High Sierra Mac. While I could do similar editing with Quicktime, when I saved the video it went from a few hundred MB to something like 2 GB! Streamclip appeared to do it without recoding the entire thing.

There's also LosslessCut. It's on the Mac App Store but I don't know if there will be a High Sierra version. The HS version is 3.56.0 at https://github.com/mifi/lossless-cut/releases

Open "Assets" and download the version for the type of Mac you have. I have an Intel Mac so I downloaded the x64 version The interface is a bit more intricate than Streamclip, but Streamclip hasn't been updated in 16 years whereas LosslessCut is ongoing. If you're thinking of the future then it may be worth learning now.

Surprisingly Streamclip is still available.

http://www.squared5.com/svideo/mpeg-streamclip-mac.html

You're welcome, and thank you for letting me know! It's nice to know when the time spent researching solutions was productive. :-)