No version of UltraEdit supported ever working without temporary files without locking the file for other applications as far as I know (since UE v10).

Working without temporary files means also working without undo feature. I cannot recommend that for small files. Use second or third option at Advanced - Configuration - File Handling - Temporary Files and set a threshold value of 4096 or 8192 to open just files with more than 4 or 8 MB without usage of a temporary file.

I edit only rarely large files. But I also do not want to wait a long time on opening and saving a large file because of usage of temporary files.

But this is the reason for the 3 options and the threshold value. With Open file without temp file but NO Prompt selected and a threshold value of 8192 KB, all files smaller 8 MB are opened with usage of a temporary file which makes it possible that other applications write on the opened file while viewed/editied in UltraEdit and undo feature can be used also within UltraEdit, and all files with 8 or more MB are opened without usage of a temporary file which of course requires to lock the file for write operations by other applications. The large file can be still read by other applications, but write is not possible anymore as long as it is opened in UltraEdit as write operations can be done on file also by UltraEdit at any time.

Well, if you just want to view the large files and not edit them, you are using the wrong application for this purpose. UltraEdit is a text editor. It is designed mainly for editing and not for viewing. Therefore UltraEdit must lock the file for write operations by other applications if opened without usage of a temporary file. Otherwise the file could be easily damaged if UltraEdit and another application write to the file at the same time. Or UltraEdit could crash because of a file change by other application on reading/writing the file content by UltraEdit itself.

Perhaps you synchronize the write operations. But UltraEdit cannot depend on write operation synchronization by the user.

For just viewing a large file it is better to use an application designed for just viewing and not an editor. For example the lister of my favorite file manager Total Commander (shareware) can display any file (text and binary) of any size for just viewing the content (and copying text to clipboard). The lister of Total Commander (short TC) is also available as free standalone tool. The lister of TC does not lock the file as there is no need to do it as the file cannot be modified, just viewed.

I have tested if a file loaded without usage of a temporary file in read-only mode results in opening it without a write lock. There are 3 methods doing that:

1. Using an alternate INI file with configuration setting Open files as read-only by default enabled.
2. Appending parameter /R on command line on starting uedit32.exe with full name of file as one more parameter.
3. Using command Read Only from menu Edit.

But UltraEdit opened the file nevertheless always with the write look respectively does not remove the write look on enabling read-only mode.

A feature request could be sent to IDM support by email for opening a read-only file without a write look even when no temporary file is used respectively change write lock on toggling Read Only mode in UltraEdit.

I have not sent such a feature request email to IDM support as I don't need it. I use Total Commander for just viewing content of a file or quickly search for a string in a file which I do not want to edit.