Promote and enhance the use of customizer in more places; open post previews in the customizer and promote customize link to top of admin bar.
The “Preview” button when editing a post is replaced with a “Preview & Customize” button, which opens the customizer with the current post's preview loaded into the customizer preview window.
A separate window is opened for each edited post being previewed, as opposed to all previews going into the same window named
When a customizer preview is opened for previewing a post, clicking the Back (Close or Cancel) button in the customizer controls, the opened window will close and the opener window will be focused on. This will rapidly take you back to the page from which you opened the preview, assuming it is still open.
It is not helpful to see in browser tabs "Customize Twenty Twelve — WordPress". This title assumes the primary purpose of the customizer is to preview themes, when in reality it seems the primary purpose is to customize your already-selected theme. In this latter case, it makes much more sense for the title to reflect the page currently being previewed. So when in the customizer, the parent document title will reflect the title of the page currently being previewed. As you navigate around the site within the preview iframe, the page's title will update. This allows you to see which page is being customized just by looking at the label on your browser's tab.
In the Admin Bar, the “Customize” link is promoted from a submenu to a top-level position right after the Edit post link.
This prominent placement of the “Customize” link in the admin bar, along with the customizer being opened when previewing a post/page, are designed to encourage the use of the customizer in many more places than just the site's front page. The Widget Customizer plugin conditionally shows control sections in the customizer based on whether or not that sidebar is currently rendered in the customizer preview. Therefore, for some customizer controls to be accessed, the user must navigate—either within the customizer or outside—to a page that has the element which a control customizes.