Don't you mean Photos? iPhoto was discontinued 10 years ago.
Regarding the start and stop buttons they will disappear when you move the cursor out of the movie/video window.
If you're doing that then try the following in order as needed:
1 - close your library and reopen it with the Option+Command keys held down to repair/restore the library. Then test a video.
2 - create a new, empty library, import a video and test to see if the problem persists in that library. If it doesn't close the test library, open your main library and test a video again.
3 - if it still does't work properly boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.
Note: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.
