There is no issue. That's just how AE samples frames at the temporal entry point of the frame and it makes perfect sense.
The CTI (current time indicator) shows you the current frame to the right of the pointer. IOW, with the CTI at frame 99 the Comp Panel is displaying the frame that ends at frame 100. Let's simplify this by changing frames to time. If the composition is 24 frames per second and the Timeline starts at frame 0:00:00:00 when you move to 0:00:00:23 and set the out point for that layer the shot will end at 24 frames or 1 second. This is a better way of doing things, and the way that editing systems have worked by default for all time because when you are editing you want to see the last frame in the sequence before the cut. So the CTI is at 23 but the duration is 24.
Things change when you split a layer. If the CTI is at 23, and the layer is 24 frames long (one second) and you split the layer the layer will split at frame 23 with the CTI now showing you the first frame of the new layer. This also makes sense but it can give you fits if you are splitting layers and don't realize that you are loosing a frame on the original when you split layers.
So in your example, the CTI at 198 is showing you the frame that ends at 199. Move the CTI to 199 and you will see black in the Comp Window if the layer ends at 199 - which makes perfect sense when you realize that time does not stop for anybody.
I am pointing out how trim to comp handles frame ranges. How it works, is indeed how it works. This has nothing to do with whether or not it makes sense.
If you edit footage, in and out points are inclusive. In at 0 and out at 0 is 1 frame. In at 0 and out at 9 is 10 frames. If you create a comp from those 10 frames and render them, you get 10 frames.
[0,9] includes frame 9.
If you trim to work area by dragging the end of the work area to where the CTI is, for example frame 9 of a 10 frame range, and you then trim to comp and then render, you get 9 frames. Before rendering, the current frame is shown as frame 9, just as it would be if you were trimming footage.
This later behavior resembles handling of ranges "exclusively".
[0,9) excludes frame 9.
But, if you do either of these actions, you will see that it is actually 9 frames that are selected, even though it is showing you the 10th frame.
1. Instead of dragging the end of work area indicator to where the CTI is, you press 'N' to move the end of the work area to the CTI.
2.After trimming go to the start of the comp, then go back to the end of the comp.
In both of the above cases, you see it is 9 frames because it shows you frame 8 instead of frame 9.
A shorter version in case it will save someone from finding this out on their own:
Move cursor to 10 and set in point
Move cursor to 19 and set out point
Create comp from footage you have frames 10-19
Move cursor to 10 drag work area start to 10
Move cursor to 19 drag work area start to 19
Trim comp you have frames 10-18
At this point the preview displays frame 19. Go to start. Go to end. The preview displays frame 18.
Unlike with trimming footage, AE doesn't add the extra frame for you at the end when you trim comp to work area.
If you use B and N to set the work area, N adds the extra frame for you in the same way that setting in and out points does.
Dragging the end of the work area to the CTI is not the same setting out point for the work area using the N key. I would not expect it to be. This functionality in AE, although the shortcuts and terms are different, work exactly the same way they do in Premiere Pro, Final Cut, Avid, Or even in my 1995 version of D–Vision, my first NLE that cost almost as much as the house I lived in at the time. I don't see an inconsisty here.
Rick, I am contributing help to fellow users of After Effects, most of whom are not you and will not have used all of the NLEs that you have used.
There seems to be a fundamental misunderstanding of the purpose of posting information in a forum.
Usually, when something is confusing to someone, it makes sense once they are no longer confused. It does not follow that they should not have been confused then.
Person A: I don't know how to say Yes in Spanish.
Person B: I don't see why you don't know. Everyone who knows how to say Yes in Spanish knows that you say Si in Spanish to say Yes.
Person A: Right. If I knew how to say Yes in Spanish, I would not be asking how to say Yes in Spanish. Thanks anyway, I guess.
For people who are or are not Rick: If you are a person who is new to after effects or you are not, it does not matter if people think that you should know something. Your confusion does not need justification, unless it's very obvious that you haven't made any effort to find the answer yourself. Your questions are valid whether or not someone would have the same questions if they had decades of experience. We are a community and we are helping each other. Regardless of experience, all of us know something others don't which is why we have formed a community to exchange information with each other.
There are no stupid questions except for why won't the IRS accept Jello as payment.