Thanks for the detailed report. This will be caused by a browser addon rather than Composr. Composr just happens to convert (almost) all JavaScript errors into alerts.
If you want to disable that, you can remove this from the HTML_HEAD.tpl in any themes running:
{+START,IF,{$NOT,{$BROWSER_MATCHES,ie}}}{+START,IF,{$HAS_PRIVILEGE,sees_javascript_error_alerts}}window.take_errors=true;{+END}{+END}
For v11 I've just added a config option to disable it, given the potential for code outside our control to create issues.
(Click to enlarge)
(Click to enlarge)
#4325 - Editing Privacy Policy text
If you want to disable that, you can remove this from the HTML_HEAD.tpl in any themes running:
{+START,IF,{$NOT,{$BROWSER_MATCHES,ie}}}{+START,IF,{$HAS_PRIVILEGE,sees_javascript_error_alerts}}window.take_errors=true;{+END}{+END}
For v11 I've just added a config option to disable it, given the potential for code outside our control to create issues.