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USWDS - Range slider: Fix webkit thumb centering after border increase - #6814
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The track border was widened from 1px to 2px in 726e775 but the webkit- specific margin-top correction on ::-webkit-slider-thumb was not updated. ::-webkit-slider-runnable-track uses content-box sizing, so its rendered height = height property + border-top + border-bottom: v3.13.0: 16px + 1px + 1px = 18px develop: 16px + 2px + 2px = 20px (track grew 2px) The -3px offset was calibrated to the 18px track. With a 20px track the centreline is 1px lower, so the thumb sat 1px below centre in Chromium and Safari at every viewport and container width. Updated to -4px and replaced the 'magic number' comment with an explanation of the geometry so the value can be maintained correctly in future. Fixes #6813
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The previous comment stated the formula as -(border-width), which implied the -4px value was simply the current 2px border. That is wrong. The correct formula is -(border_width + 2px), where the +2 is a fixed geometric constant. Verified empirically: border=1px (v3.13.0) → -3px border=2px (current) → -4px Updated the comment to state the formula accurately and explain that the constant 2 is border-independent.
jonathanbobel
approved these changes
Aug 14, 2026
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Summary
Fixed off-center range slider thumb in Chromium and Safari by correcting the
margin-topoffset on::-webkit-slider-thumbto account for the track border width increase introduced in #6811.Breaking change
This is not a breaking change.
Related issue
Closes #6813
Related pull requests
Depends on / follows #6811 (range slider track border width increase).
Preview link
Preview link: N/A
Problem statement
The range slider track border was widened from
1pxto2pxin #6811 (commit726e7750a), but the webkit-specificmargin-topcorrection on::-webkit-slider-thumbwas not updated to match.::-webkit-slider-runnable-trackusescontent-boxsizing, so its rendered height equalsheight + border-top + border-bottom:16px + 1px + 1px = 18px(centerline at9px)16px + 2px + 2px = 20px(centerline at10px)The
-3pxoffset was calibrated to the 18px track. With a 20px track, the thumb sat 1px below center in Chromium and Safari at every viewport and container width.Solution
Updated the
margin-topoffset from-3pxto-4pxon::-webkit-slider-thumb. Replaced the// magic numbercomment with a full geometric explanation so the value can be maintained correctly when the track border changes in the future.This is a one-line Sass change. No markup, JavaScript, or settings are affected.
Testing and review
::-moz-range-thumb, which has its own rule and was not changed).