From the video I see that the editor freezes after maximizing it, but it doesn’t explain how the file could be corrupted. I can only see the start of the file source code, did you check the whole code? Is is possible that the bootstrap is only pasted in the document at the top. It is possible that you for example dragged the css file from the explorer in the code editor, this would paste the content from the file at that position in the code.
Please send us the log file for the freezing problem, see How to send us the Wappler debug log