I use FFMPEG X11grab for video recording. At the same time, I use Jack + Ardour for audio recording. I would like to know if I can automatically sync the two at record time instead of edit time. I don’t know how the “video monitor” thing works since the docs are incomplete on that point. Currently, I am using Debian 9, so I actually use Ardour 3 + Libav-tools, but I can upgrade those if I need to.
The video monitor aspect of ardour is for playback of video, not recording. If you want to sync audio and video recording in separate tools (hardware or software), you probably need to ensure that both tools receive timecode from the same source. It is unlikely (but possible) that a software tool like ffpmeg x11grab will generate or use timecode. Sync generally requires all participants to provide/use a common time reference.
My current workaround is to use FFMPEG for all V/A record and extract the audio for edit