SUMMARY
After installing the 2026-09 Preview Update (KB5124010) (build 26200.9550) on 9/23/2026, all dropdown/combobox lists in MP3tag in Dark mode started showing a white solid rectangle when hovering with the mouse or navigating with the keyboard Up/Down arrows. The proper dark-mode selection highlight is completely missing.
The problem disappeared after I uninstalled the September preview.
It returned immediately after installing the 2026-10 Security Update (KB5127648) (build 26300.9711) on 10/14/2026.
This only happens in Dark mode (Color Mode = Dark or Default while Windows is dark). Light mode is unaffected.
This affects any dropdown menu, thus being typeable menus, or MP3Tag fixed non-typeable menus. Any listing items accessible through dropdown menus are affected.
Program general menus ("File", "Edit", "View", etc.) are unaffected.
Affected controls
All Win32 combobox-style dropdowns: Tag Panel fields, Genre, Custom List Values, Action parameters, Action (Quick), etc.
Reproduction Steps
- Set MP3tag → Options → Appearance → Color Mode = Dark.
- Click any Tag Panel field that shows a dropdown arrow.
- Hover with the mouse or use ↑ / ↓ keys.
- Result: highlighted row becomes a solid white rectangle (text is also white/light) → unreadable.
Expected behavior
Blue background + light text for the hover/keyboard selection state (as it worked before the September 2026 preview).
Technical Details
The affected controls are standard Win32 COMBOBOX (with CBS_DROPDOWN / CBS_DROPDOWNLIST) and the popup ComboLBox window that renders the list.
The regression is isolated to the popup list’s selection/hover state. The closed combobox itself still renders its current selection correctly.
MP3tag already applies dark-mode theming (owner-drawing where necessary + DarkMode_* / Explorer themes via SetWindowTheme). The symptom matches known post-2025/2026 dark-mode combobox regressions in other Win32 applications after similar servicing updates, where the selection rect / highlight brush for themed combobox lists stops receiving the correct dark-mode color (or falls back to a white/no-color state).
Relevant indicators:
• Only the list portion (ComboLBox) is broken.
• Hover + keyboard navigation both affected.
• Closed combobox selection still works.
• Problem appeared with 2026-09 Preview (KB5124010, 26200.9550) and returned with 2026-10 Security Update (05127648, 26300.9711).
• Light mode path is unaffected.
Workarounds
• Set MP3tag Color Mode to Light (or Default + switch Windows to Light).
• Uninstalling the September 2026 preview previously restored correct behavior.
Environment
• MP3tag version: 3.36.1 x64, and previous older MP3Tag versions, as well
• Windows: Windows 11 Enterprise 26H2, build 26300.9711
• Updates that triggered the bug:
◦ 2026-09 Preview Update (KB5124010) – build 26200.9550 – installed 9/23/2026 (symptom appeared)
◦ 2026-10 Security Update (05127648) – build 26300.9711 – installed 10/14/2026 (symptom returned)
• MP3tag installation: Portable
• Color Mode: Dark (forced), Default (when Windows theme is set this way)
• Display scaling: Both 100% at 1920x1080 (with external Samsung screen attached to HDMI port) and 125% at 1920x1080 (laptop lid screen)
• Windows theme: Dark
Attachments
- Screenshot of a dropdown in normal (non-hovered) state.
- Screenshot while hovering or using arrow keys (white rectangle).
- "winver" window with exact build string.
Additional notes
• This only affects the list portion of comboboxes. Fixed menus, main window, and most other controls continue to work in dark mode.
• The issue is 100% reproducible on this machine and followed the exact same pattern with the September 2026 preview.
