Written in C#.

Aegisub

This worked well on 3.2.2 Ubuntu 20.10. Recommended.

First import video with:

aegisub-3.2 ourbigbook-parent.mkv

They don't have an aegisub executable without the version number. Amazing.

If you already have a subtitle file that you want to edit, then just pass it on as well:

aegisub-3.2 ourbigbook-parent.mkv ourbigbook-parent.ass

Ctrl + P: play and pause video.

Ctrl + 3: set current substitle start time.

Ctrl + 4: set current substitle end time.

Enter: finish editing the current entry and start a new one.

Gaupol

 0  0 

github.com/otsaloma/gaupol

Good shortcuts and user experience.

No waveform viewer: github.com/otsaloma/gaupol/issues/49 so unusable.

kitone/subtitleeditor

 0  0 

github.com/SubtitleEdit/subtitleedit

Not to be confused with Subtitle Edit.

As of 0.54.0 this feels featureful, but extremely buggy or lacking UI obvious enhancements that would be simple to implement, and offer huge value:

fundamental not defined out of box, e.g. Tiny forward/backwards. You can define them yourself, but they should be provided.
github.com/SubtitleEdit/subtitleedit/issues/4976 can't export SubRip?
can't interact with video on waveform?

It is hard to understand how that project reached this weird featureful but crappy state. Feels like they just gave push permission to a bunch of random people.