Steps to reproduce:

Create a new project
Import the attached Repro_MalformedFilterValue.cs into Assets/
Open any Editor window so a live UI Toolkit panel exists (the Inspector is fine)
Run QA > F-09 malformed filter value throws forever from the menu bar
Observe the Console

Actual results: ArgumentOutOfRangeException is thrown once on the first style resolution and twice on every re-resolution afterwards, for as long as the element exists. Nothing is logged at import time.

ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
at System.Collections.Generic.List[T].get_Item (System.Int32 index)
at UnityEngine.UIElements.StyleSheets.StylePropertyReader.ReadInt (System.Int32 index) [StylePropertyReader.cs :: 246]
at UnityEngine.UIElements.StyleSheets.StylePropertyReader.ReadListUnmanagedFilterFunction (...) [StylePropertyReader.cs :: 1183]
at UnityEngine.UIElements.ComputedStyle.ApplyProperties (...)
at UnityEngine.UIElements.VisualTreeStyleUpdaterTraversal.ProcessMatchedRules (...)

Expected results: A malformed style value is reported once at import, with the file and line, the way other invalid USS values are. It should never produce an unhandled index exception, and never one that repeats on every style resolution.

Reproducible with versions: 6000.6.2f1, 6000.7.0b2, 7000.0.0a1

Can’t test with versions: 6000.0.84f1, 6000.3.24f1 (filter and backdrop-filter properties do not exist)

Tested on (OS): macOS 15 (Darwin 25.6.0), Apple Silicon