Mail acts differently on my MB Air

I am not sure how to explain this question. It is an odd thing and I can't think of quite the right words. But here goes.

I have an iMac on my desk, and also a MB Air. I am using Yosemite on both of them, 10.10.5. On my iMac Mail works as it always has in the sense that I can respond to a mail by clicking the respond tab at the upper end of the mail (arrow pointing left). When the mail opens, and I start typing a reply, then the mail I am replying to opens up in a new window. The listing of mails in the In Box are still seen underneath this window. On my iMac, I can go back and forth between these windows. By this I mean that I can click on the underlying window of the In Box and read some of the text in the mail I am responding to, then go back to the open reply mail I am working with. I hope this is clear. I mean to say that I can toggle between the two windows.

On my MB Air, I cannot do this. If I reply to a mail, it opens in a new window as described above, but then the underlying list of mails in the In Box are grayed out and if I click on them there is no response. As well, the open window of the reply mail cannot be moved on the screen. I cannot move it to the left or right or down. It is just stuck there. I can continue to type into the reply and send it OR I can cancel and then go back and read the original mail again.

I know this sounds confusing, but this is the best I can put it – that the open reply window does not allow any window switching or any movement on the screen.

I have checked settings and there is no difference between the two computers that I can see, so must have to do with the Air architecture I am thinking.

If anyone has any idea about this, I would love to hear from them. Makes using Mail on the Air a frustrating experience.

Richard