without immediately putting his mask back on even though staff members were nearby and he could still be contagious, according to medical studies of the virus timeline. At that point, he filmed the video, which was quickly uploaded to Twitter. A separate video, set to triumphal music, showed Marine One’s return and his saluting pose, and was posted online within an hour of his landing.The president looked stronger than he did on Friday when he was first taken to the hospital, but he did appear to breathe heavily once reaching the top of the White House stairs. He has been taking steroids that are known to produce a feeling of energy, even exhilaration, while suppressing pain or discomfort.After largely laying off Twitter, his favorite form of communication, for three days, Mr. Trump woke up Monday morning and began blasting out a string of messages in all capital letters in machine gun fashion shortly after 6 a.m., amplifying campaign messages like “LAW & ORDER. VOTE!” and “SAVE OUR SECOND AMENDMENT. VOTE!” By afternoon, he added, “Don’t be afraid of Covid. Don’t let it dominate your life.”