Steps to reproduce:

1.  Open a new project
2.  Add a scoped registry (Edit > Project Settings > Package Manager - Scoped Registries section)
3.  In Package Manager window add any unsigned package by name

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Actual results: 
A pop up for unsigned package has appeared which is not rendered properly and no buttons are visible in the window

Expected results: 
A pop up for unsigned package has appeared and rendered correctly with visible buttons

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Reproducible with versions: 
6000.3.0a5, 6000.3.1f1, 6000.4.0b1, 6000.5.0a3

Can’t test with versions: 
6000.2.15f1, 6000.3.0a4 (no Missing Signature pop up)

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Tested on (OS): 
Ubuntu 22.04
Ubuntu 24.04

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Notes:

•  Works as expected on MacOS and Windows
• Closing the pop up window still imports the package