Dark mode: ComboBox / ComboLBox popup list selection shows white (no-color) rectangle instead of proper highlight after 2026-09 Preview (KB5124010) and 2026-10 Security Update (KB5127648 / 26300.9711)

[spdking01]

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

1. Set MP3tag → Options → Appearance → Color Mode = Dark.
2. Click any Tag Panel field that shows a dropdown arrow.
3. Hover with the mouse or use ↑ / ↓ keys.
4. 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

1. Screenshot of a dropdown in normal (non-hovered) state.
2. Screenshot while hovering or using arrow keys (white rectangle).
3. "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.

[ohrenkino]

On my PC it looks like this:

[LyricsLover]

I can reproduce the behaviour here with Win11 Pro, 26H2 (Build 26300.9711):

[ohrenkino]

I chose this as colour setting:

[LyricsLover]

+1

I only switched to Dark Mode in Mp3tag to try it out. I don't usually use Dark Mode.

[ohrenkino]

I checked both:
Windows: bright, MP3tag: dark
Windows: dark, MP3tag: dark

and in both cases I have the blue background.

[LyricsLover]

JFYI:
I have just installed the latest available graphics card driver from Nvidia (version 617.42).
This has not changed anything.

[ohrenkino]

My PC uses Intel HD Graphics.

[Florian]

I already have a local fix for that which will be included with the next beta. Let's hope that this works on all systems. Have I mentioned that dark mode for Win32 apps is one gigantic hack?

[spdking01]

UPDATE TIMELINE (from Windows Update history / Get-HotFix)
2025-11-11 25H2 feature update installed
2026-09-08 KB5124008 (26200.9445), KB5126052 (.NET), .NET 8/9 security updates
2026-09-14 KB5129195 security update (26200.9457)
2026-09-22 KB5124009 (servicing stack)
2026-09-23 KB5124010 preview (26200.9550 on 25H2) <- bad
2026-09-30 26H2 feature update + KB5121794
2026-10-07 KB5124010 uninstalled, reboot -> build 26300.9457; white highlight gone <- fixed
Multiple earlier reboots did not fix the problem; uninstalling KB5124010 did.

WHAT WAS TESTED / RULED OUT

• Clean portable Mp3tag copy, other folder, version 3.35: same bug while KB installed.
• Intel graphics driver 31.0.101.2145 and 31.0.101.2141: bug present with the KB under both;
  with the KB removed, normal under 2141 and still normal after reinstalling 2145.
• Forcing the NVIDIA GPU for Mp3tag: no change.
• Windows theme changes: no change. Other apps: not affected.
• A minimal test app with a dark-mode ComboBox on the same PC: normal blue highlight.
• Modules loaded in the Mp3tag process outside the Windows folder: only Mp3tag.exe.

COMBOLBOX OBSERVATIONS

• Window class "ComboLBox" becomes visible only while a dropdown is open (several hidden instances exist). Styles seen: 0x44808043 and 0x44A08043; ExStyle 0x80 / 0x88.
• Neither LBS_OWNERDRAWFIXED (0x10) nor LBS_OWNERDRAWVARIABLE (0x20) is set, so the selection fill appears to be drawn by the system, not by the app.
• System colors read from a separate PowerShell process looked normal (COLOR_HIGHLIGHT #0078D4, COLOR_HIGHLIGHTTEXT #FFFFFF); this does not show what the Mp3tag process received.

FILE VERSIONS IN THE GOOD STATE (KB5124010 uninstalled, build 26300.9457)
user32.dll 10.0.26100.8875 (2026-09-08)
uxtheme.dll 10.0.26100.8875
win32u.dll 10.0.26100.9444
dwmapi.dll 10.0.26100.8875
themeui.dll 10.0.26100.8875
d2d1.dll 10.0.26100.8875
dwrite.dll 10.0.26100.8875
dwmcore.dll 10.0.26100.9278
comctl32 v6 (WinSxS):
6.0.26100.9278 created 2026-09-08 (active, loaded by Mp3tag)
6.0.26100.9549 created 2026-09-22 (inactive; probably shipped with KB5124010)
Baselines saved: 3,627 System32 DLLs (version/size/date) and 15 theme files
(Resources\Themes, SHA256). After installing an Intel driver update only 7 Intel DLLs changed;
Theme files and comctl32 versions stayed identical.

The KB release notes do not mention controls or dark mode. A relationship with comctl32
9549 is only a lead. The app may use undocumented dark-mode behavior in uxtheme, so this
could also be a compatibility regression.

Maybe we can gather more information to help you and ease a solution for this problem.

At least for me, this problem only surfaced for the first time to me after the September preview of the October Patch Tuesday pack.

[LyricsLover]

@Florian

Have I mentioned that dark mode for Win32 apps is one gigantic hack?

Not yet

[Florian]

@spdking01

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.

This should be fixed with Mp3tag v3.37-beta.3. Please let me know if this fixes the issue for you.

[LyricsLover]

It works fine here now:

What does it look like to you @spdking01?

[spdking01]

Hi Florian,

Sorry for the late reply. My email provider locked me out of my account for several days, so I couldn't log in to the forum to answer you.

I can confirm that 3.37 Beta 3 fixed the problem. The white rectangle on the highlighted item in ComboBox dropdowns (ComboLBox) in Dark mode is gone, and the selection is blue again, as it should be.

Thank you very much for the quick fix. Less than a day is impressive. And no, I didn't know the Windows dark theme was such a hack. Thanks for the explanation!

Best regards!

[spdking01]

The fix Florian provided solved the issue!

I am with latest October Patch Tuesday installed in my PC and with 3.37 Beta 3 the normal/correct behaviour returned to normality.

I could not replay earlier because my e-mail provider locked me out for reasons I can only figure and, thus, I could not login at the forum to reply you, guys.

[windev]

Hello Florian,

A fellow dev here fighting with Win32 dark mode... Can you please describe your fix? Not setting the "DarkMode_Explorer" (SetWindowTheme call) helps with the text visibility, but now scrollbars' dark mode is lost, they stay bright. How did you resolve this trade-off? Thank you! Note that I'm forcing dark style independent of Windows mode.

[Florian]

I had to change the SetWindowTheme call to only modify the Scrollbar class. So changing

SetWindowTheme(hwndListBox, L"Explorer", nullptr);

to

SetWindowTheme(hwndListBox, L"Explorer", L"Scrollbar");

did the trick.

[windev]

Totally ignored the third parameter... That solved the issue. Thank you very much!