refactor: single scrollbar add and named slack in getHeadersWidth

[ghiscoding]

verified by Copilot using GPT-5.3-Codex

Port bug fix from 6pac/SlickGrid PR 6pac/SlickGrid#1274 into slickgrid-universal

Resolves the Q16 investigation item from the quirks triage (discussion 6pac/SlickGrid#1247): getHeadersWidth() added the vertical-scrollbar width to the active band twice — once in the conditional gutter-attribution block (from 6pac/SlickGrid#1260, gated on !autoHeight), and once more, unconditionally, in the band-finalization block. Two generations of code expressing the same intent.

Why the duplicate add contributes nothing

The band widths already carry the classic +1000 fixed slack (applied to the left band and again to the return value), which is what actually guarantees the two invariants the header width serves:

1. the header band's scroll range covers the body viewport's scroll range (header width is the header/body scroll-sync floor), and
2. column drag-reorder has room past the last column.

A second ≤17px scrollbar term riding on a 1000px slack cannot be load-bearing. It was also wrong-shaped: the conditional add correctly skips autoHeight grids (which have no vertical scrollbar), while the unconditional one gave them a gutter for a scrollbar that doesn't exist.

The change

• Exactly one scrollbar add remains — the documented, conditional gutter attribution.
• The two unconditional duplicate adds are deleted.
• The 1000 literals are promoted to a named module constant, HEADER_WIDTH_SLACK, with a comment stating what the slack is for — the intent is now explicit rather than folklore.

Net effect on computed widths: the header container gets ≤17px narrower (and autoHeight grids lose the phantom gutter). The container is clipped, and both scroll-range floors retain the full 1000px slack, so nothing is user-visible.

This also simplifies the #1238 rework: the ViewportMgr's computeHeaderWidths currently mirrors the double-add bit-for-bit; after this lands it mirrors one sane formula.

Test

cypress/e2e/headers-width-scroll-sync.cy.ts — a regression pin, not a bug repro (there is no fail-on-unfixed leg, and per that, no temporary example page). Self-hosting harness with three grids — plain, frozenColumn: 1 (right band scrolls), and autoHeight — asserting for each: header scroll range ≥ body scroll range, header scroller reaches the body's scrollLeft at full right scroll (no clamping), and the last column's header stays pixel-aligned with its body cells there. Verified to pass on the pre-change build (baseline) and on the refactored build, so any future width-formula change that breaks scroll sync fails this spec.

Full cypress suite: all specs passed — 652 tests, 650 passing, 2 pre-existing skips (4m51s).

(Quirks-triage follow-up: with this, every actionable item from discussion 6pac/SlickGrid#1247 is resolved on master — remaining entries are the documented keep-as-is decisions and the Q32 product call.)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (e8ca333) to head (de8bcdb).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2717   +/-   ##
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  Coverage   100.0%   100.0%           
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  Files         200      200           
  Lines       25515    25507    -8     
  Branches     9024     9012   -12     
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- Hits        25515    25507    -8     

Flag	Coverage Δ
angular	100.0% <ø> (ø)
universal	100.0% <100.0%> (ø)

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commit: de8bcdb