I’ve had this happen on three older Sage/Roots sites so far.
In all cases, upgrading WP to 5.3 broke the WP Editor by making it impossible for WP to load all of the relevant CSS.
The fix is to go into:
/lib/cleanup.php
and comment out the line that hooks the cleanup function roots_clean_style_tag into the style_loader_tag filter like so:
//add_filter('style_loader_tag', 'roots_clean_style_tag');
Before:
After:
I hope that helps someone!