Protractor only sends DOWN key successfully when the test window is opened

In my Protractor test, I run this command at some time:

browser.actions().sendKeys(protractor.Key.DOWN).perform();

It works very well when the test window is opened, but if I'm working on another window at the same time and the test window is reduced, Protractor doesn't send the key.

Do you know why, and do you know how I could solve this problem?