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Prefab modifications aren't saved when "Auto Save" is enabled

Scene Management


Reproduction steps:
1. Open the user's attached "" project
2. Open the "SampleScene" Scene
3. Open the "Test" Prefab Asset
4. Check the "Auto Save" tick-box at the top of the Scene view
5. Click the "Edit Grid" button in the Inspector window inside of "TestGrid (Script)" Component
6. Left-click on the Scene view window to draw line blocks
7. Exit the "Test" Prefab Asset
8. Observe the Scene view window

Expected result: All drawn blocs are saved
Actual result: The minority of drawn blocks are saved

Reproducible with: 2019.4.33f1, 2020.3.24f1, 2021.2.5f1, 2022.1.0a16

- Unchecking the "Auto Save" tick-box and saving the prefab manually doesn't reproduce the issue

  1. Resolution Note:

    In order to support drag-painting blocks you need to set the GUIUtility.hotcontrol property while painting (See

    The Prefab Mode checks if the hotcontrol is set and delays autosaving until it is released again.

    Example for your editor script:

    private void OnSceneGUI()
    int controlID = GUIUtility.GetControlID(FocusType.Passive);

    if (GUIUtility.hotControl == 0 && Event.current.type == EventType.MouseDown && Event.current.button == 0)
    GUIUtility.hotControl = controlID;
    Debug.Log("Grab hotcontrol");

    if (GUIUtility.hotControl == controlID && Event.current.type == EventType.MouseUp && Event.current.button == 0)
    GUIUtility.hotControl = 0;
    Debug.Log("Release hotcontrol");


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