Scrollbar slider thumb becomes nearly invisible at the bottom edge of webview editor tabs in Light Mode (macOS)

When using macOS with a Light theme (e.g., Light Modern), the vertical scrollbar slider thumb in webview editor tabs (such as Claude Code or custom extension panels) fades and becomes virtually invisible near the bottom boundary of the window.

Even when forcing scrollbar visibility via "editor.scrollbar.vertical": "visible" and applying high-contrast color overrides in workbench.colorCustomizations (scrollbarSlider.background / scrollbar.shadow), the bottom portion of the thumb continues to fade out against the light canvas background due to webview frame mask clipping and macOS overlay scrollbar interactions.

Steps to Reproduce
Launch VS Code on macOS set to Light Mode / Light Modern theme.

Open any extension webview editor tab (e.g., Claude Code, Markdown Preview, or Interactive Notebooks).

Scroll down toward the bottom of the document or chat panel.

Observe the vertical scrollbar thumb as it approaches the bottom edge.

Expected Behavior
The scrollbar thumb should maintain uniform opacity, contrast, and visibility along the entire length of the track, including at the bottom boundary.

Actual Behavior
The top of the scrollbar thumb remains dark/visible, but the bottom half rapidly fades into near-total transparency, blending into the white background and making it extremely difficult to locate.

VS Code version: Code 1.136.1 (a44adf7f53e00964ab890f9f8758a334f1fc15bc, 2026-09-03T05:06:41Z)
OS version: Darwin arm64 23.6.0
Modes:
Remote OS version: Linux x64 5.14.0-427.42.1.el9_4.x86_64

System Info

|Item|Value|
|---|---|
|CPUs|Apple M1 (8 x 2400)|
|GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x106b [Google Inc. (Apple)], DEVICE=0x0000 [ANGLE (Apple, ANGLE Metal Renderer: Apple M1, Version 14.6.1 (Build 23G93))], DRIVER_VENDOR=Apple, DRIVER_VERSION=14.6.1 *ACTIVE*
Machine model name: MacBookPro
Machine model version: 17.1
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: enabled_on
trees_in_viz: disabled_off
video_decode: enabled
video_encode: enabled
webgl: enabled
webgpu: enabled
webnn: disabled_off|
|Load (avg)|2, 2, 2|
|Memory (System)|16.00GB (0.05GB free)|
|Process Argv|--crash-reporter-id dbda8ff8-52e4-4ebd-99e2-73f7a9b84fbd|
|Screen Reader|no|
|VM|0%|

|Item|Value|
|---|---|
|Remote|SSH: ra4|
|OS|Linux x64 5.14.0-427.42.1.el9_4.x86_64|
|CPUs|AMD EPYC 9634 84-Core Processor (168 x 0)|
|Memory (System)|1511.04GB (1353.10GB free)|
|VM|0%|
Extensions (8)

Name|Identifier|Author|Version
---|---|---|---
Claude Code for VS Code|anthropic.claude-code|Anthropic|2.1.261
SVN|johnstoncode.svn-scm|johnstoncode|2.17.0
XLSX, CSV, TSV & Markdown Editor|muhammad-ahmad.xlsx-viewer|muhammad-ahmad|1.9.98
Jupyter Keymap|ms-toolsai.jupyter-keymap|ms-toolsai|1.1.2
Remote - SSH|ms-vscode-remote.remote-ssh|ms-vscode-remote|0.128.0
Remote - SSH: Editing Configuration Files|ms-vscode-remote.remote-ssh-edit|ms-vscode-remote|0.87.0
Remote Explorer|ms-vscode.remote-explorer|ms-vscode|0.5.0
R Syntax|reditorsupport.r-syntax|REditorSupport|0.1.4

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binariesv615:30325510
nativeloc1:31344060
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dwoutputs:31242946
copilot_t_ci:31333650
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pythonrdcb7:31342333
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cloudbuttont:31379625
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diffpatch-lysithea-24-production:31579162
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conptydll_true:31498968
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control_6dc23131:31576178
ei9d7968:31496641
chat:31457767
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i2gc6536:31499202
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ddid_t:31478206
hmra_i5g22:31518061
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cp_cls_t_1082:31535311
logging_enabled_new:31498466
db5d2638:31499441
jb_cp_cls_t_632:31543129
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ha629193:31508444
jh5f2457_c:31540921
jbcp_cls_pctr_t:31531130
cp_intellij_t_nes:31548657
gf14b233:31526830
ahp-both-windows:31556933
ihg5j128:31534457
9gg7f176:31542112
sandbox-ui-on-2:31543161
enable_editor_pane_layout:31569726
allow-none:31555437
36h42362:31564511
c7c27ce7:31554789
1h923230:31564177
0dg73464:31554319
treatment-23-1:31555779
unuse_dynamic_mcp:31555281
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maidictation:31565333
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vrbsty_tr:31561058
session-archive:31558132
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mangle-name-control:31572633
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```