User Steps
The current process to edit a page (or post) in Customizer, is as follows:

1. From the front end site page or post the user wants to edit, the user clicks Customize to open Customizer. As expected, the page appears in the preview window in Customizer.

However to actually edit the page in customizer the user has 3 more steps:
2. Click on Pages in the left navigation pane
3. Click on Name of Page
4. Click on [Open Editor]

Recommended Usability Enhancement
After clicking on Customize from a specific page or post in the customizer have that page/post already open/expanded in the left navigation pane, so the user only has to select to open editor.

See screen shots