If I were to guess you were using a Mac, would I be correct?
If so, this is a bug that is supposed to be fixed in an update "very soon" according to Adobe. Usually when the AE team says "very soon" they actually do mean it's coming fairly quickly. So, in the meantime, you might need to use CC 2015.3 for a few days.
I'm certainly using a bunch of Macs and have tested on 2x Mac Pros and 1x MacBook Pro to make sure I'm not crazy. Do you have any documentation from Adobe concerning the bug? In terms of tracking back - my files are all CC 17 - can you work backwards?
Ah found the documentation:
WorkaroundTry one of the following:
- Reduce the vertical size of the Timeline panel. You can also dock a different panel under the Timeline, such as the Render Queue, and raise the vertical divider between them.
- Hide the Timeline panel by enabling Full Screen in the Preview panel, or maximizing the Composition panel during the preview (` key, or in the Composition panel menu).
- Hide the preview-time indicator (the red line that moves across the Timeline panel during a preview), but it is complex. You need to zoom-in on the Timeline and then move the view away from the area being previewed.
In terms of tracking back - my files are all CC 17 - can you work backwards?
Yes! You can save as version 13 out of the CC 2017 release (which is AE version 14.0). That file can then be opened by CC 2014 (AE version 13.0-13.2), CC 2015 (AE version 13.5-13.7), and/or CC 2015.3 (AE version 13.8).
Everybody who is having this issue, please install the latest update that came out last night: After Effects CC (November 2017) Bug-Fix Update Is Now Available
This update is 14.0.1 and should be a bug fix for this and several other disturbing issues - especially for Mac users. It does not fix all the bugs that are new with the CC 2017 release (Known issues in After Effects CC 2017), but it does fix some issues that have been plaguing After Effects since the first parts of the new architecture showed up in the CC 2015 release.
If you can't see the update yet, you may need to wait a while until you see it as it can take up to 24 hours to populate an update across all the data centers. You may need to log out, relaunch the Creative Cloud app, and long in again.