Steps to replicate the issue:

• Install TemplateStyles and CodeMirror
• Set $wgTemplateStylesDefaultNamespace = 8;
• Create a template style in the MediaWiki namespace, e.g. MediaWiki:Test.css
• Change its content model to Sanitized CSS
• Use <templatestyles src="Test.css" /> and make sure it loads correctly
• Use Ctrl+click on the src attribute value in CodeMirror

What happens?: Nonexistent page Template:Test.css opens.

What should have happened instead?: The correct page, MediaWiki:Test.css, should have opened.

Software version: MediaWiki 1.45.3 (d2cfa30). TemplateStyles 1.0 (e898cce)

Other information: This happens because this line assumes the namespace is 10, even though it's configurable. The fix would probably involve injecting $wgTemplateStylesDefaultNamespace as a JS config var.