"workbench.experimental.modernUI" silently enabled via A/B experiment, breaks titleBar/statusBar/activityBar rendering (transparent, no fallback) with Quiet Light theme

[GuyunLuke]

Type: Bug

Does this issue occur when all extensions are disabled?

Yes (tested with code --disable-extensions, issue persists)

VS Code Version

1.131.0

OS Version

Windows 10

---

Summary

After a spontaneous restart (not a manual theme change), the title bar, status bar, and activity bar suddenly rendered as fully transparent/white, even though the active color theme (Quiet Light) and all computed CSS color variables remained correct. The issue persisted across:

• Full uninstall + reinstall of VS Code (including manually deleting the install directory)
• Renaming %APPDATA%\Code to force a completely fresh user profile/state
• code --disable-extensions
• code --disable-gpu
• Clearing all caches (Cache, CachedData, Code Cache, GPUCache)
• Turning off Settings Sync
• Setting "window.titleBarStyle": "custom"
• A completely clean settings.json with no colorCustomizations, no autoDetectColorScheme, nothing theme-related besides "workbench.colorTheme": "Quiet Light"

None of the above resolved it. The root cause was eventually traced via source inspection.

Root Cause

workbench.experimental.modernUI was somehow set to true, despite the user never touching it and it never appearing anywhere in settings.json (user-level, workspace-level, or profile-level).

This setting triggers isFloatingPanelsEnabled() in layout.ts, which adds a floating-panels class to .monaco-workbench. This class then activates a CSS rule in workbench.desktop.main.css:

```css
.monaco-workbench.floating-panels .part.activitybar,
.monaco-workbench.floating-panels .part.statusbar,
.monaco-workbench.floating-panels .part.titlebar {
background-color: transparent !important;
border-color: transparent !important;
}
```

This !important rule overrides the theme's actual colors (confirmed via getComputedStyle, values below), with no fallback background/material rendered behind it, resulting in a broken, flat-white appearance for the title bar, status bar, and activity bar — regardless of which theme is active.

Evidence

Computed CSS variables were correct at all times:
```
--vscode-editor-background: #f5f5f5
--vscode-sideBar-background: #f2f2f2
--vscode-activityBar-background: #ededf5
--vscode-titleBar-activeBackground: #c4b7d7
--vscode-statusBar-background: #705697
```

But the actual status bar element showed:
```js
{
class: 'part statusbar',
inlineStyle: 'background-color: rgb(112, 86, 151); color: rgb(255, 255, 255); ...',
computedBackgroundColor: 'rgba(0, 0, 0, 0)' // transparent, overridden by !important rule above
}
```

Confirmed via direct stylesheet inspection that the matching !important rule came from workbench.desktop.main.css, scoped under .monaco-workbench.floating-panels.

Suspected Cause of the Setting Being Enabled

The user never manually set workbench.experimental.modernUI. It's suspected this was silently enabled via VS Code's built-in A/B experimentation service (exp-tas.com), which is active by default whenever telemetry is not fully disabled. If so, an experimental, visually-breaking feature was rolled out to a user cohort without any opt-in, notification, or safe fallback rendering.

Steps to Reproduce

1. Be assigned (silently, via A/B experiment) or manually set "workbench.experimental.modernUI": true
2. Restart VS Code
3. Observe that title bar / status bar / activity bar render as transparent/flat with no background, regardless of active color theme

Expected Behavior

• Experimental settings that visually break the UI should not be silently enabled for users without explicit opt-in.
• If floating-panels mode is active, there should be a proper background/material rendered behind the "transparent" regions instead of leaving them blank.
• getComputedStyle should not report accurate theme colors on an element whose real rendered color is completely different due to an !important override — or at minimum this override should be discoverable via normal theme/settings UI.

Workaround

Manually add to settings.json:
```json
"workbench.experimental.modernUI": false
```

VS Code version: Code 1.131.0 (e4c7e7b, 2026-07-28T10:51:25Z)
OS version: Windows_NT x64 10.0.19045
Modes:

System Info

Item	Value
CPUs	12th Gen Intel(R) Core(TM) i5-12600KF (16 x 3686)
GPU Status	2d_canvas: enabled GPU0: VENDOR= 0x10de, DEVICE=0x2d05 [NVIDIA GeForce RTX 5060], DRIVER_VENDOR=NVIDIA, DRIVER_VERSION=32.0.16.1062 ACTIVE GPU1: VENDOR= 0x10de, DEVICE=0x2d05 [NVIDIA GeForce RTX 5060], DRIVER_VERSION=32.0.16.1062 GPU2: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.19041.7548 Machine model name: Machine model version: direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_graphite: disabled_off trees_in_viz: disabled_off video_decode: enabled video_encode: enabled webgl: enabled webgpu: enabled webnn: disabled_off
Load (avg)	undefined
Memory (System)	63.79GB (43.76GB free)
Process Argv	--disable-extensions --crash-reporter-id 1f9030f2-e668-4277-a325-cbdde37f2a7f
Screen Reader	no
VM	0%

Extensions (13)

Name	Identifier	Author	Version
Claude Code for VS Code	anthropic.claude-code	Anthropic	2.1.222
Git History	donjayamanne.githistory	donjayamanne	0.6.20
GitHub Actions	github.vscode-github-actions	github	0.32.3
Chinese (Simplified) (简体中文) Language Pack for Visual Studio Code	ms-ceintl.vscode-language-pack-zh-hans	MS-CEINTL	1.131.2026072717
Python Debugger	ms-python.debugpy	ms-python	2026.6.0
Python	ms-python.python	ms-python	2026.4.0
Pylance	ms-python.vscode-pylance	ms-python	2026.3.1
Python Environments	ms-python.vscode-python-envs	ms-python	1.36.0
Remote - SSH	ms-vscode-remote.remote-ssh	ms-vscode-remote	0.124.0
Remote - SSH: Editing Configuration Files	ms-vscode-remote.remote-ssh-edit	ms-vscode-remote	0.87.0
PowerShell	ms-vscode.powershell	ms-vscode	2025.4.0
远程资源管理器	ms-vscode.remote-explorer	ms-vscode	0.5.0
Claude Notifier	singularityinc.claude-notifier	SingularityInc	3.6.1

A/B Experiments

vsliv368:30146709
binariesv615:30325510
nativeloc1:31344060
dwcopilot:31170013
dwoutputs:31242946
copilot_t_ci:31333650
e5gg6876:31282496
pythonrdcb7:31342333
6518g693:31463988
aj953862:31281341
4f60g487:31327383
envsactivate1:31551504
gh487529:31544284
cloudbuttont:31379625
3efgi100_wstrepl:31403338
ec5jj548:31422691
cp_cls_t_966_ss:31526232
4je02754:31466945
c3h7c220:31478652
cp_cls_c_1081:31454833
conptydll_true:31498968
e9c30283:31461165
test_treatment2:31471001
46204921:31447328
idci7584:31464702
e3e4d672:31494082
ei9d7968:31496641
chat:31457767
7e187181:31503455
ghj88844:31499326
ec1eg703:31520046
ddid_t:31478206
hmra_i5g22:31518061
7df3h592:31512476
cp_cls_t_1082:31535311
logging_enabled_new:31498466
j0d79568:31499440
jb_cp_cls_t_632:31543129
56dj4588:31512888
32d76977:31512328
ha629193:31508444
cdk-lw-on:31524445
f9bf7985:31543584
a1ije391_t:31540920
g4a0e106:31529594
8hfba999:31537718
jbcp_cls_pctr_t:31531130
61138546:31518536
cp_intellij_t_nes:31548657
jf4hg949:31526829
ahp-both-windows:31556933
h561g132:31533256
session_8:31532895
ihg5j128:31534457
f412h606:31551416
9gg7f176:31542112
5fe6f778:31554331
allow-none:31555437
6i51c363:31551401
c7c27ce7:31554789
8hc4g144:31558143
1532g621_copy:31554320
treatment-23-1:31555779
unuse_dynamic_mcp:31555281
0d8dfbc3:31560381
a1hcc538:31559322
vrbsty_fls:31561059
session-archive:31558132

[gjsjohnmurray]

/duplicate

[vs-code-engineering]

Thanks for creating this issue! We figured it's covering the same as another one we already have. Thus, we closed this one as a duplicate. You can search for similar existing issues. See also our issue reporting guidelines.

Happy Coding!