Steps to reproduce:

Create a new project from any template
Import the attached PollingTool.cs into an Assets/Editor folder and wait for the compilation to finish
In the Project window, right-click the Assets folder and select Create > Scripting > MonoBehaviour Script
Press Enter to keep the default name and wait for the compilation to finish
Observe the Editor

Actual results: The Editor crashes during the script reload

Stack overflow.
   at UnityEngine.DebugLogHandler.LogFormat(UnityEngine.LogType, UnityEngine.Object, System.String, System.Object[])
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
   at System.Threading.TimerQueue.FireNextTimers()
Obtained 1 stack frames
 #0 __pthread_kill

Expected results: The script reload finishes and the Editor keeps running

Reproducible with versions: 7000.0.0a5, 7000.0.0a8

Not reproducible with versions: 6000.0.84f1, 6000.3.24f1, 6000.7.0b5, 7000.0.0a1

Tested on (OS): macOS 26.6.2

Notes:

Some runs crash with EXC_BAD_ACCESS (address 0x0) instead of the stack overflow; full traces in the comments
Also crashes with Task.Run, Awaitable.BackgroundThreadAsync(), a Process.OutputDataReceived handler or a FileSystemWatcher handler, with JsonUtility instead of Debug.Log, and on entering Play Mode when Play Mode reloads the domain
Only reproduces with the CoreCLR scripting runtime: 6000.x and 7000.0.0a1 (Mono) do not crash
With the timer at 5 seconds instead of 20 ms the crash is intermittent: 2 of 3 runs crashed within 10 script reloads (at reloads 3 and 6), 2 crashes in 19 reloads in total