Fix padding issue to correct the wiggle.

## Problem

Content would "wiggle" left-right in flow mode when fit to width.

## Solution

Padding was being scaled, making the DOM elements too wide when the scale-to-fit value was >1. Don't do that.

## Tests

Added.

## QA

Open the scroll list demo, make the browser window as wide as possible, set the fit mode to "width". You shouldn't be able to scroll left-right at all by default (un-zoomed).