Files with a full path of exactly 259 characters cannot be opened

Description of the Issue

In Notepad++ v7.5.9 (64-bit) under Windows 10 Version 1803, paths (including filename + extension) of exactly 259 characters cannot be opened. Files with longer or shorter paths can be opened normally.

Steps to Reproduce the Issue

1. Create a file with a full path length of exactly 259 characters.
2. Try to open the file with Notepad++ (either by double clicking the file, right-clicking and using 'Open with' or 'Edit with', or opening Notepad++ and using the File>Open menu).

Expected Behavior

The file should be opened and its contents displayed.

Actual Behavior

Notepad++ opens with the default new file 'new 1'.

Debug Information

Long paths (>260 characters) are enabled in the registry.

Names of files with longer paths (exceeding Windows' default 260 character limit) are displayed in all caps and abbreviated with ~ in the Notepad++ tab, even if the filename itself is quite short.

Reproduce the issue on Windows using a file whose full path is exactly 259 characters, then trace the file-open path used by the File>Open menu and shell entry points. No source file or test is named in the report, so identify the relevant path-handling code and add coverage showing that the file opens and its contents are displayed.