To reproduce:
1. Open attached project ("case_1212854.zip")
2. From menu bar select Window > HoverTest
3. Hover an element in a list
4. Expand the custom window by dragging the right corner outwards

Expected result: No elements are in a hover state
Actual result: A single element is in a hover state

Reproducible in: 2019.2.19f1, 2019.3.0f5, 2020.1.0a20
Could not test in: 2017.4.36f1, 2018.4.16f1 (UIElements library not available)

Notes:
1. The issue doesn't reproduce if the cursor leaves the window entirely before resizing it
2. Only reproducible by expanding the window to the right or bottom side