The Triggers Grid looks like this:
I can still select a row (mouse or keyboard) and see the trigger source below though.
It looks ok here.
Try right-clicking in the grid and choosing "Reset Columns"
That did it. However, I have no idea why the setting was so crooked.
It looks like the first column got extremely wide. I saw a tiny horizontal scrollbar in your screen shot.
Do you have multiple displays of different scalings? That could have been a factor in how it happened - settings not getting scaled correctly to the current display. Normally, the grid should handle that but maybe something went wrong there.
That's right, there was a very small scroll bar. However, I also tried moving it all the way to the right. But nothing became visible.
I only use Toad in a Windows VM that uses only one screen.