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Fixed in Built-in



Found in [Package]


Issue ID




[UIToolkit] ArgumentExceptions are thrown and spammed after transition with empty property

Package: UI Toolkit


UIToolkit throws an ArgumentException followed by a continuous neverending stream of Exceptions when a VisualElement that is not the last element is removed from a Hierarchy and also when it has child elements that have transition USS directives that reference non-existent properties

Reproduction steps:
1. Open the user's attached project ""
2. Open the "Sample Scene"
3. Enter Play Mode
4. Click the Button at the top
5. Observe the Console window

Expected result: ArgumentExceptions are not thrown and spammed after transition with empty property
Actual result: ArgumentExceptions are thrown and spammed after transition with empty property

Reproduces on: 2022.1.0b14, 2022.2.0a9
Does not reproduce on: 2021.2.15f1
Could not test 2019.4.36f1, 2020.3.32f1 due to package errors when downgrading

ArgumentException: An item with the same key has already been added. Key: UnityEngine.UIElements.StylePropertyAnimationSystem+ElementPropertyPair
at System.Collections.Generic.Dictionary`2[TKey,TValue].TryInsert (TKey key, TValue value, System.Collections.Generic.InsertionBehavior behavior) [0x001cc] in <06a8633d56964cb4a0c4050a8ba5ed74>:0
at System.Collections.Generic.Dictionary`2[TKey,TValue].Add (TKey key, TValue value) [0x00000] in <06a8633d56964cb4a0c4050a8ba5ed74>:0

-Restarting the Editor/Player stops the Exceptions from being spammed in the Console
-This issue happens in both the editor and in players (tested on Android, iOS)
-This issue does not pop up if the element removed is the only/final element of the hierarchy of its parent
-This issue does not pop up if the property referenced in the transition actually is set. To see this, change minimalReproUSS.uss to:
.miau-button {
background-color: transparent;
transition: background-color 250ms;}

Comments (2)

  1. Soraphis_

    Mar 15, 2023 12:04

    is still an issue in 2022.2 HDRP

  2. ainisciurlys

    Oct 26, 2022 10:40

    The fix was validated in 2023.1.0a2

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