TBF - whatever you thinkj of Trump he's not the worst person the Royal have had to meet, indeed the Queen has met many world leaders including Robert Mugabe of Zimbabwe, President Nicolae Ceaușescu and First Lady Elena Ceaușescu of Romania, Emperor Hirohito and Empress Kōjun of Japan and later Emperor Akihito, Chinese President Xi Jinping as well as former Chinese President Hu Jintao, King Abdullah of Saudi Arabia, Russian President Vladimir Putin who she shared a carriage with, and there are numerous others.
The Queen even admitted to seeing Nicolae Ceaușescu one morning and quickly darting in the oppositte direction, the Ceaușescu's were later executed by the people of Romania.
The visits by the Japanese Emperors were also difficult as the Queen is Commander and Chief of the British military and lots of former British PoW's lined the streets of London in order to turn their back on the Japanese Emperor, due to the horrific treatment of prisoners of war by Japan during WW2.
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The Queen also actually visited Russia in 1994 when relations were better than now, even though the Russian Royal Family (the Romanovs) who were murdered during the Russian Revolution were cousins of the British Royals.
The Queen also met with Sinn Fein leaders as part of the Northern Ireland peace process, despite Lord Louis Mountbatten ( a well loved member of the Royal Family), being murdered by the IRA, although Prince Philip just walked past them and refused to shake their hands.
Trump is fairly easy to deal with by comparison, and Trump was brought up to love the Queen by his late mother, so was very respectful and not a problem, indeed Prince Charles even sat him down for tea and tried to change his mind on the need for action in relation to climate change.
US Presidents have never been a problem since the first state visit by Woodrow Wilson in December 1918, and the Royals always receive the US President with a great deal of warmth and hospitality.