Incorrect curve visualization in Dopesheet for identical slot alpha keys

[misaki-eymard]

Description:
When two adjacent slot alpha color keys with exactly the same value are set to a Bézier curve, a curve is still displayed between the keys in the Dopesheet view, indicating interpolation—even though the curve handles are perfectly horizontal.

Unfortunately, we have not yet identified a reliable way to reproduce this issue from scratch. At present, it can only be reproduced using the Spine project file provided by a user.

Expected behavior:
If two consecutive keys have identical values and their curve handles are horizontal, no curve should be displayed between them in the Dopesheet view—even if the keys are set to Bézier.

Steps to reproduce:

1. Download the attached .zip file: 993-slot-alpha-curve-issue.zip
2. Open the included Spine project file using Spine 4.3.53-beta.
3. Switch to Animate mode. In the Dopesheet view, observe that a curve is displayed between the slot alpha key at frame 10 and the key at frame 70. Then switch to the Graph view and confirm that the curve handles for these keys are perfectly horizontal:

4. Select the key at frame 10 and change it to a linear curve. This removes the issue in the Dopesheet view; however, a curve now appears in the Curves view. In the Curves view, no curve should be shown when the selected key and the following key have identical values:

5. Select the key at frame 10 again and change it back to a Bézier curve. The curve between frame 10 and frame 70 reappears in the Dopesheet view, while the curve in the Curves view disappears.

Version where this issue was observed:
4.3.53-beta

This issue was reported in the following forum thread:
https://esotericsoftware.com/forum/d/29950-slot-color-incorrect-behaviour-both-in-editor-and-unity-beta-runtime