Reproduction steps:
1. Open the attached project ("case_1336406.zip")
2. Open the "SampleScene": Assets > "SampleScene"
3. Open and dock the Package Manager window beside the Console window
4. Hide the Package Manager window by selecting the Console window
5. Select the "DelayCallTest" GameObject in the Hierarchy window
6. Repeatedly click the "Execute" button, in the Inspector window

Expected result: Total methods count, assigned to the "EditorApplication.delayCall" delegate, doesn't change, when the "delayCall" delegate is invoked
Actual result: Total methods count, assigned to the "EditorApplication.delayCall" delegate, doubles, once the "delayCall" delegate is invoked.

Reproducible with: 2019.3.0a7, 2019.4.28f1, 2020.3.11f1, 2021.1.10f1, 2021.2.0a19
Not reproducible with: 2018.4.35f1, 2019.3.0a6

Note:
- Method ("ScrollIfneeded") is being repeatedly added to the "EditorApplication.delayCall" delegate when the "delayCall" delegate is Invoked.
- The issue isn't reproducible when the Package Manager isn't docked.
- The issue seems to be caused only by the Package Manager window. Preferences, Project settings windows, etc. had no side effects.