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Yes, the problem was in my custom JS syntax definition, a long time ago I added highlighting for nested HTML in JavaScript template literals and apparently broke something…
1. Textastic builtin JS syntax

2. My custom JS syntax

Hang for me, maybe it related with my custom CSS/JS syntax definitions. I'll let you know later.
Alexander have you tried opening syntax_test_html.html from my archive?
Only on latest Textastic beta
Unfortunatly Textastic not supported Subline Text syntax definitions V2
Yes its related with my custom html syntax highliting
Maybe it's the similar bugs?
The problem lies in "clear_scopes" settings
Customer support service by UserEcho
Fixed