fix(grid): eliminate empty space after last column and scrollbar not reaching the end - 21.2.x

[Zneeky]

Closes #17428

Description

Fixes the Grid's horizontal scrollbar leaving empty, unscrollable space after the last
column, with the scrollbar thumb never reaching the end of its track.

Root cause: igx-horizontal-virtual-helper (and its vertical counterpart) used a shared
overflow: auto on both axes. The horizontal helper's height is pinned to exactly the
native scrollbar thickness, so once the real horizontal scrollbar claims that space,
clientHeight becomes 0. Since overflow-y was also auto, the helper's 1px-tall
placeholder content then always "overflows" that 0px client height, causing the browser
to spawn a phantom vertical scrollbar. That phantom scrollbar eats ~16-17px of width from
the helper, shrinking its effective scrollable range below the real content width — so
the grid can never scroll far enough to bring the last column fully into view.

Fix: constrain each virtual helper to scroll only along its intended axis —
overflow-y: hidden on the horizontal helper and overflow-x: hidden on the vertical
helper (defensive, symmetrical fix for the same latent failure mode) in
_igx-vhelper.scss.

Motivation / Context

Reported bug: opening almost any grid sample with a horizontal scrollbar (e.g. the Grid
Filtering dev sample) and scrolling all the way to the right leaves a gap after the last
column, and the horizontal scrollbar thumb stops short of the track's end.

Type of Change (check all that apply):

• Bug fix
• New functionality
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes)
• Documentation
• Demos
• CI/CD
• Tests
• Changelog
• Skills/Agents

Component(s) / Area(s) Affected:

Grid (and other grid-based components sharing igxFor virtualization: Tree Grid,
Hierarchical Grid, Pivot Grid) — horizontal/vertical virtual scrollbar helpers.

How Has This Been Tested?

• Unit tests
• Manual testing
• Automated e2e tests

Manually reproduced and verified via the Grid Filtering dev sample: measured
clientWidth/scrollWidth of igx-horizontal-virtual-helper before and after the fix,
and confirmed scrollLeft now reaches exactly scrollWidth - clientWidth, with the last
column ("Index") flush against the grid's right edge.

Test Configuration:

• Angular version: 22.1.x (also verified applies cleanly to 22.0.x and 21.2.x)
• Browser(s): Chrome (Windows)
• OS: Windows

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code
• This PR includes API docs for newly added methods/properties
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes
• This PR includes behavioral changes and the feature specification has been updated with them
• Accessibility (ARIA, keyboard navigation, focus management) has been verified (no ARIA/keyboard impact — style-only fix)

[Copilot]

Pull request overview

Fixes an issue in grid virtualization scroll helpers where a phantom cross-axis scrollbar could appear and shrink the effective horizontal scroll range, leaving empty space after the last column and preventing the horizontal scrollbar thumb from reaching the end (issue #17428).

Changes:

• Constrains the vertical virtual helper to vertical scrolling by hiding horizontal overflow.
• Constrains the horizontal virtual helper to horizontal scrolling by hiding vertical overflow.