Comment 4 for bug 1299079

The position of the "Apply" button already implies that it applies to every setting in the panel, including UI scale. If it didn't apply to UI scale, we would have to move the button and the scale controls so that they obviously didn't relate to each other.

It's also generally a bad idea for changing a setting to alter the UI of the setting itself.

For those two reasons, I agree with Iain that this setting should not be instant-apply. So that can be changed straight away. That leaves the problem of displaying a preview of the new setting, so I'll work on that next.