I'm not seeing a problem with this. I can type some letters in the Search box, and after matches are displayed, I can still move the mouse around the screen and get back the matches to select something.
Must be something specific to you computer.
What operating system are you using? What kind of mouse?
I am using Windows 10 with either a microsoft wired mouse or a logitech wireless. I've used MC on multiple computers over the years, and multiple versions. I don't believe this is new or unique behavior to me. It happens when the search bar is selected, but the mouse is not near that area. For instance:
enter the search bar, type something, select a result from the drop down list. Now what you have selected gets put in to the search box and is selected
move the mouse into the "data" screen to use as a "finger on the page" to help look thru the results. Search bar is still active and the rsults are still highlighted. Use the keyboard to enter another search and get drop down results. move the mouse back to the results, and they will disappear before the mouse pointier gets to them