When selecting text in a textarea or input element such as the demo on MDN, more text than selected is copied to primary selection.
For example, when selecting "something" and pasting the primary selection via middle-click in another application, "Write something here" is pasted instead. When pasting stuff into e.g. IRC, when multi-line text is copied, this might be a privacy issue as well.
It seems it's always exactly 100 chars before and 100 chars after the selected text which is getting copied. When pressing ctrl-c, the primary selection is set correctly.