Very minor issue due to unlikely and invalid input sequences, one of many that could be found with a few minutes of fuzzing, I assume.

Application: wincompose.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.OverflowException
   at System.ParseNumbers.StringToInt(System.String, Int32, Int32, Int32*)
   at System.Convert.ToInt32(System.String, Int32)
   at WinCompose.Settings.GetGenericSequenceResult(WinCompose.KeySequence, System.String ByRef)
   at WinCompose.Composer.AddToSequence(WinCompose.Key)
   at WinCompose.Composer.OnKeyInternal(WinCompose.WM, WinCompose.VK, WinCompose.SC, WinCompose.LLKHF)
   at WinCompose.Composer.OnKey(WinCompose.WM, WinCompose.VK, WinCompose.SC, WinCompose.LLKHF)
   at WinCompose.KeyboardHook.OnKey(WinCompose.HC, WinCompose.WM, IntPtr)

This can be reproduced by enabling "advanced unicode input" and typing:
composeud8341d11eEnter

(This particular combination was a mistake while I was testing how best to type the G clef character 𝄞: it is not in the basic multilingual plane, so it is usable for some (modern) apps as U+1d11e and others only as a utf-16 surrogate pair \ud384\udd1e. For the record, the former works fine in WinCompose, which is preferred.)

In case this self-evident stack trace is not enough, I can probably provide dumps and other proper debugging info if necessary. Not that it should matter for this particular issue, but I am on Windows version 10.0.18363 (Build 18363).

Am I correct in guessing that this overflow is happening here, and that it would be averted by comparing m.Groups[1].Value against Int32.MaxValue before that line and returning false if it's too large?

Thank you for an excellent piece of open-source software.