Description of the Issue

This was originally reported on the Community Forum https://community.notepad-plus-plus.org/post/66533
Thanks to @pryrt and Alan Kilborn for providing details and @sasumner for testing.
Some discussion was in #9732.

Steps to Reproduce the Issue

1. Press Ctrl+N to create new file; new1 tab appears
2. Press Ctrl+Alt+S to invoke Save As dialog
3. Check the Append extension checkbox:
   
4. Set the file name to anything without extension (e.g. z):
   
5. Click on Save button using left mouse button

Expected Behavior

File is saved with extension as z.txt

Actual Behavior

File is saved without extension as z

Note:
In step 5 if mouse click is replaced by

• Enter key - works as expected (.txt extension added automagically)
• Alt+S - doesn't work (no extension added)

Debug Information

Notepad++ v8 (64-bit)
Build time : May 27 2021 - 22:10:00
Path : E:\Installs\npp\npp.8.0.portable.x64\notepad++.exe
Command Line :
Admin mode : OFF
Local Conf mode : ON
Cloud Config : OFF
OS Name : Windows 10 Enterprise (64-bit)
OS Version : 2009
OS Build : 19043.985
Current ANSI codepage : 1252
Plugins : mimeTools.dll NppConverter.dll NppExport.dll