Trimming a layer (rather than time stretching) by holding opt/alt with "Responsive Design — Time" enabled appears to be broken
I am using the "Responsive Design - Time" functionality for the first time in After Effects and unless I'm misunderstanding the functionality, trimming, rather than time stretching, appears to be broken right now.
I have a composition that contains a video clip that is replaceable via the essential graphics panel. The composition has a protected intro and outro. In the nested composition, I need to be able to extend the composition length via trimming, rather than stretching, in order to preserve the speed of the replaceable video clip contained within it.
According to the help page for the responsive design - time feature, "To trim the in or out point of the layer instead of time-stretching, hold the Option (macOS) or Alt (Windows) key and drag the in and out points."
But when I attempt to lengthen the composition while holding alt, After Effects does not let me. Oddly I can adjust the positions and lengths of the protected portions of the composition by holding alt and dragging them, but I cannot actually figure out if this is affecting anything.
Is this a bug, or am I misunderstanding the functionality here?
I think what might be happening here is a collision between 2 features. The Media Replacement feature has some hard rules around time, dimensions, and frame rate that were needed to make it work. Whatever layer you use to make the Media Replacement control in the EGP, essentially sets the "container" of time, dimension, and duration for any media that is replaced later downstream.
If I'm understanding what you're describing correctly, then that's likely why you aren't able to trim that layer.
Duration is a tricky one though, because if you use a still image as the MR control which has no intrinsic duration, then the duration is set by the parent comp.
The comps that are created when you replace media via Essential Properties can be accessed to then make custom changes to but within these constraints, which in this case I think would require you to make timing changes with stretch or expressions.
Alternatively, nesting the MR Contol in another comp and adding the Responsive Design-Time at that point in the hierarchy might be a possible workaround.
It would be easier for me to try to figure out a workaround with a sample project and a screen recording of what you're trying to do though. I'm going to email you now and hopefully, we can find something that works!