I would recommend checking out LosslessCut. Behind the curtain it runs ffmpeg, so you should be able to find the perfect command.
In the features list:
View ffmpeg last command log so you can modify and re-run modify recent commands on the command line
Actually that might not have been done to deliberately disrupt your flow. Culling elements that are outside of the viewport is a technique used to reduce the amount of memory the browser consumes.