Open the "Components" or "Patterns" tab of the editor's Library panel, opening the panel first when it is closed (never toggling an open panel shut), and wait for the tab's content to render. Open a page in the editor first.
Examples
When I open the "Components" tab in the Canvas Library
And I open the "Patterns" tab in the Canvas Library
When we open the "Components" tab in the Canvas Library
Given I open the "Patterns" tab in the Canvas Library
And we open the "Patterns" tab in the Canvas Library
Pattern
When(/^(I |we )*open the "(Patterns|Components)" tab in the Canvas Library$/, …)
Part of @webship-js/drupal-cms-steps (github.com/webship/drupal-cms-steps), a step pack for Webship-js 2.0.x.