Step1. drag the first item (*.mp4) from Playlist to Timeline.
Step2. drag the second item (*.mp4) from Playlist to Timeline,
to the same track and the right-hand-side of the first item,
then I saw a wierd behavior of Timeline, that is,
the right-half-part of the 2nd item becomes still frames,
and shows a white block on right-top of this 2nd clip on the Timeline.
Today, I’ve download two short video (*.mp4) to test the timeline again,
they proved the ShortCut is OK 
!!
I found that my 2nd item on Yesterday actually
have some still scene on the right-half-part,
and it was created by Filmora 9’s Screen Recorder,
so It seems something wrong on Filmora 9, not ShortCut.
(Filmora 9 is still a baby software, maybe it will be OK next year)
(Filmora 9 Screen recorder save 35 seconds mp4 to 102 seconds)
(Filmora 9 cannot Export 2 short mp4 created by its Screen Recorder, buggy, now)
(I’ve posted a Crash report to Filmora’s forum 2 days ago, but no one care till now)