Imagine that you download a .odt/docx/pdf form with embedded font in LibreOffice in 2025. You start to type some text... And font start to saturate FPU ports (i.e. div/sqrt) in specific pattern. Meanwhile some tab in browser measures CPU load or port saturation by doing some simple action, and capture every character you typed.
iirc browsers fuzz the precise timing of calls for exactly this reason already?