attachHandles() {
      const classes = [STATE_CLASS_MODS.leftHandle, STATE_CLASS_MODS.rightHandle];
      const spanStart = self._spans[0];
      const spanEnd = self._spans.at(-1);

      classes.forEach((resizeClass, index) => {
        // html element that can't be encountered in a usual html
        const handleArea = document.createElement("area");

        handleArea.classList.add(resizeClass);
        index === 0 ? spanStart.prepend(handleArea) : spanEnd.append(handleArea);
      });
    }
Attach resize handles to the first and last spans. `area` is used to be less possible to be
in user's document. They are not fully valid inside spans, but they work.