paulirish

What forces layout / reflow

All of the below properties or methods, when requested/called in JavaScript, will trigger the browser to synchronously calculate the style and layout*. This is also called reflow or layout thrashing, and is common performance bottleneck.

Generally, all APIs that synchronously provide layout metrics will trigger forced reflow / layout. Read on for additional cases and details.

Element APIs

Getting box metrics

• elem.offsetLeft, elem.offsetTop, elem.offsetWidth, elem.offsetHeight, elem.offsetParent

ericandrewlewis

The WordPress Customizer

The WordPress Customizer is an interface for drafting changes to content while previewing the changes before they are saved. This is an alternative to the "save and suprise" model of changing settings without knowing what exactly will happen.

The customizer can be accessed in the admin interface under Appearance > Customize.

#19909 is the trac ticket that introduced the Customizer during the 3.4 release cycle.