I was doing a fresh install of a Windows 11 VM so snaphotted it after the install and tried installing an old calibre version after setting MS Shell Dlg 2 to Courier New in the registry.
- installed calibre 4.23.0 and I was able to select MS Shell Dlg 2 and the interface changed to Courier New.
- Reverted to the snapshot
- installed calibre 5.44.0 and I was able to select MS Shell Dlg 2 and the interface changed to Courier New.
- Reverted to the snapshot
- installed calibre 6.2.1 and MS Shell Dlg 2 was not visible in the font menu. Selected Courier New and the interface changed to Courier.
- Reverted to the snapshot
- installed calibre 4.23.0 and selected MS Shell Dlg 2 as the interface font. Updated to 5.44.0 and the font selection was still there. Updated to 6.2.1 and the ability to select MS Shell Dlg 2 in the font list disappeared and the interface font changed to Arial (New).
And yes, I had way too much time on my hands today. The only way to restore the interface font after the 6.2.1 update was to locate the actual font file and select it by name.
Edit: Did some browsing and ran into several items stating the QT6 was no longer able to select Microsoft's logical fonts (MS Shell Dlg 2 being one of those).