Blender Git Loki

Python API: new RNA property `Screen.is_scrubbing`

This commit adds a new read-only boolean property `Screen.is_scrubbing`.

The related property `Screen.is_animation_playing` is set to `True` in
two situations:

- Animation is actually playing (for example via the Play button in the
timeline)
- The user is scrubbing through time (in the timeline, dopesheet, graph
editor, etc.)

To distinguish between these two cases, the property
`Screen.is_scrubbing` has been added.

Concept approved by @brecht.