Bug 1020862 - Calculator Visual Refresh follow-up

I am just realizing that I meant
`parseFloat(window.getComputedStyle(this.display, null).getPropertyValue('font-size'));`
not
`parseFloat(((9 - this.display.textContent.length) / 2.7) * 10)`
And no AFAIK we can't have `||` because we don't have a default value to use in case this returns an empty string..