I have sought help from the folks here a few times with quick and accurate resolutions, thank you for that. I wanted to bring this to people’s attention who might have the same issue and share my resolution.
I use the standard ‘Popular Posts’ widget, no CSS or other modifications, and found that when I click on a post’s linked title it opens correctly with no issue. However, when I clicked on the same post’s thumbnail image it opens correctly, in a new window.
Within the widget code, it was being told to do exactly the problem I described, to open the thumbnail link in a new window.
<div class='item-thumbnail'> <a expr:href='data:post.href' target='_blank'>
I simply removed the section target=’_blank’ saved the template and reloaded the page to verify.
Bam! Fixed it!
Hope this helps.