⟨SHY⟩
Invisible until the line runs out; then it surfaces as a hyphen, in the right place.
The soft hyphen marks where a word may be divided, showing nothing for as long as the division isn't needed. When the line runs out, the browser breaks there — and only there — with a hyphen.
It is the answer for long words in narrow columns: antidisestablishmentarianism, electroencephalogram. Seeded with ­, the word splits gracefully at any width instead of bursting the box.
Bare: electroencephalogram (bursts the narrow column)
With ­: electro­encephalo­gram (breaks where the author allowed)