Fix T84486: NLA, disable "Sync Length" after split

Turn off "Sync Length" when splitting an NLA strip.

The NLA Strip-Action Clip has a setting called "Sync Length" that is on
by default and helps to update the length of the clip to the current
actions keyframes so the strip shows the entire actions stored
animation.

When you split one of these strip-action clips in the NLA to trim it
shorter or to move it somewhere else in the NLA tracks to blend or work
with, the "Sync Length" setting stays checked on.  You can have many
strips in the NLA that all look to the same action, if you split one
strip , you now have two strips showing or linked to the same action.

To see or edit keyframes on a strip, you enter tweak mode. When you exit
tweak mode, if "Sync Length" is active on the strip-action clip
settings, the strip length is changed/reset to match the action keys.
When you have many clips, this causes all of them to evaluate and update
no matter where they are in the NLA.  This destroys/undoes all the user
work to trim down and place the clips in the NLA.

**Description of the proposed solution:**

This patch/change would turn off, the "Sync Length" setting when the
split tool was used on a strip/action clip to help protect the users
choice to trim and keep the clip that length. It doesn't change the
ability to turn the setting back on or off, it just makes sure that the
user doesn't accidentally or unknowingly loose work.

The same process happens when an action is created that has a "manual
range" set that is different than the length of the actions start-end
keyframes. It makes sense to do the same thing for the split tool.

**Alternative solutions:**

While the user could know this and turn off this setting by hand, it is
easy to forget and or the user might think that it only happened to the
one clip they were editing and not realize it happened to all the
trimmed versions, changing the users choice without the user knowing it
happened.

**Limitations of the proposed solution:**

It only fixes the split tool, if another tool was created and some how
impacted the clip length we would need to have that tool also take the
sync length into account.

Reviewed By: sybren

Maniphest Tasks: T84486

Differential Revision: https://developer.blender.org/D10168

1 files changed, 5 insertions, 2 deletions

diff --git a/source/blender/editors/space_nla/nla_edit.c b/source/blender/editors/space_nla/nla_edit.c
index 1376dade659..28041c0389f 100644
--- a/source/blender/editors/space_nla/nla_edit.c
+++ b/source/blender/editors/space_nla/nla_edit.c
@@ -1397,8 +1397,11 @@ static void nlaedit_split_strip_actclip(
     nstrip->actstart = splitaframe;
   }
 
-  /* clear the active flag from the copy */
-  nstrip->flag &= ~NLASTRIP_FLAG_ACTIVE;
+  /* Make sure Sync Length is off. With that setting on, entering and exiting tweak mode would
+   * effectively undo the split, because both the old and the new strip will be at the length of
+   * the Action again. */
+  strip->flag &= ~NLASTRIP_FLAG_SYNC_LENGTH;
+  nstrip->flag &= ~(NLASTRIP_FLAG_SYNC_LENGTH | NLASTRIP_FLAG_ACTIVE);
 
   /* auto-name the new strip */
   BKE_nlastrip_validate_name(adt, nstrip);