Michael Ng

Nah, I'm also encountering same issue in 4.3.12. If you open many images (min. 5 in my cases), either file size is big or dimension is large, the first image will always be unable to use select rectangle tool. Moreover, if you go to edit the last image you put into paint.net and save, then return to the beginning page, there are high chance that this situation will reoccur.