When you choose to edit a template part (select one and click "Edit"), you edit it in an isolated view, with a clearing surrounding the template part, and responsive canvas resize handles letting you preview any arbitrary size canvas:

This experience of previewing your site's responsiveness is arguably more ergonomic than the tablet and mobile options in the preview dropdown, which do not feature those resize handles:

The main challenge is, you can only get access to these preview handles if you click to edit an entity in isolation, e.g. a template part.

Proposal: we should enable these resize handles through a new option in the preview dropdown, called "Responsive":

This would also allow us to surface the options for tablet and mobile, those two options would simply become preset widths for this mode. I.e, click View > Mobile, and you'd see the mobile size. Within some decided upon size margin, scaling up would move the option to tablet, or simply to responsive.