Some UX changes to make it clearer what mode user is in and faster to get back to editing #2
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Hi @simonxeko, lovely extension, works really well. I made a couple of UX tweaks for personal use, which you might want to consider merging if you like them.
A quick preview below:

In summary, the modifications do the following:
They allow you to dismiss the preview when clicking on the actual preview box, which takes you back to the composer with the cursor focused where you had left off. This seems like it would be more natural and expected behaviour, since users might find themselves wanting to edit the preview and click on text they would expect to be editable. (the preview button also focuses the composer input element now, which is a bit smoother for workflow).
The preview button is now highlighted with the Button--danger style (red in the default template), to make it very obvious that the preview is currently active to avoid some potential confusion for users. I found it wasn't necessarily clear for less technical users, whether it was wysiwyg or a passive preview.
Along the above lines, while in preview mode, a grey 'eye' icon from font-awesome is rendered as a watermark behind the preview window. I tested this on mobile and desktop and it seemed fine, but your mileage may vary on whether you like this. I found it a useful secondary indicator that the text was an uneditable preview.
Do please let me know if any queries or feedback.
Ps.: Am a bit new to github, so I just saw the webpack dependency upgrade in composer.json - sorry, that should not have been included.