Gmail shortcut keys overridden by DevTools shortcut keys

I can't get Gmail shortcut keys to perform the Gmail actions because (I think) the DevTools actions for that shortcut are performed instead. For instance, to mark an email as read is Ctrl+Shift+I, but this opens a DevTools inspection window. This inspection window opens on any page, not just the Gmail page, so I believe it is Chrome.

I have enabled keyboard shortcuts in Gmail settings.
I have gone to Chrome > settings > extensions and turned off developer mode.
I have restarted Chrome after doing this.

I can't be the only one who has this trouble, but I cannot find any reference to it elsewhere. Any further help is appreciated.