When LibreOffice is configured to use its own file dialog implementations with KDE (4.x) (on gentoo Linux x86-64), text within the file name cannot be selected with the mouse.
The problem has been present since at least LibreOffice 220.127.116.11, and is unchanged up to at least 18.104.22.168.
1. In Tools -> Options, LibreOffice -> General, enable "Open/Save dialogs": "Use LibreOffice dialogs"
2. Open a file dialog, i.e. File -> Open... or File -> Save as...
3. Select a file in the upper pane
4. Try to select text in the "File name:" field below, using the mouse
No selection paradigms work (drag, double-/triple click). Instead, any click in this field will drop down the field's history list.
Selecting text using the keyboard (arrow keys, hold Shift to select) works correctly.
Selecting parts of the file name by dragging or multiple-clicking with the mouse should work as in any other editable text field. The drop down should only open when clicking the drop hint at the far right of the text field.
The related gentoo bug is https://bugs.gentoo.org/show_bug.cgi?id=415367
The first comment of this gentoo bug also has a screencapture of the UI in question.
I'll be happy to provide additional information, or try things out.
The issue is still unchanged with LibreOffice 22.214.171.124, with KDE 4.11.2 / Qt 4.8.5.
Reproducible with a master build (Build ID: 8102d45911bf3c47ce7ee15d3db89b0024c3bff8) with '--enable-kde4'.
Fixed by http://cgit.freedesktop.org/libreoffice/core/commit/?id=a62a80ac6cc0e337f4ff6d862bccc13c720d41ac