I am not an English native speaker and this problem is hard to describe in words. Forgive me for my bad English.
I posted a video record of the issue in url bilibili-click here and youtube-click here, please take a look, this problem really bothered me a lot.
How this bug occurred:
I was using UE 4.26 for this whole project and when I recently updated my NVIDIA driver from 460.89 to 461.09, Windows10 crashes everytime when I tried to open UE4.26 editor, so I think I met a driver issue and downgrade my driver from 461.09 back to 460.89 and this error happened. Whenever I tried to select on the pull-down menu of the editor toolbar, the bug occurred and I cannot click to select. However, the pull down menu from left clicking on anything in the content menu works perfectly fined. I have tried to reinstall the whole engine and reinstall the driver but this error still occurs. And when I create a new project with UE4.26 this weird bug didn’t happen. However, I have never change a single line of engine source code.