[clever-octopus]

The remaining time from your current visa is added to your next leave. You can also apply for ILR 28 days before your 5-year anniversary, so you can apply for ILR in December 2019.

surely this would have caused a great deal of trouble for people otherwise?

Only when someone has entered the UK significantly later than their visa issue date (this is pre-BRP days when you received the full visa in your passport). With the advent of the BRP and 30-day vignette, it's more closely controlled and there's less room for massive differences in visa expiry vs renewal periods