fix(splitter): preserve pane sizes after collapse

[viktorkombov]

Closes #17545

Description

This PR prevents IgxSplitterPane from losing its configured size after being collapsed and expanded.

Previously, collapsing a pane reset its siblings to size="auto", which caused horizontal panes to lose their previous width. The updated behavior:

• Preserves pane sizes and min/max constraints.
• Clears only temporary drag sizes.
• Allows fixed-size siblings to fill the released space while a pane is collapsed.
• Handles both runtime and initially collapsed panes.

Testing

Added coverage for horizontal and vertical splitters, fixed-size panes, initial collapse, and temporary drag-size cleanup.

Motivation / Context

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:

How Has This Been Tested?

• Unit tests
• Manual testing
• Automated e2e tests

Test Configuration:

• Angular version:
• Browser(s):
• OS:

Screenshots / Recordings

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• 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 (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them
• Accessibility (ARIA, keyboard navigation, focus management) has been verified

[Copilot]

🟡 Changes recommended

The new unit tests rely on strict offsetWidth/offsetHeight equality, which is likely to be flaky due to rounding/layout differences across runs and environments.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates the Splitter collapse/expand behavior so IgxSplitterPane preserves its configured sizes (and min/max constraints) instead of resetting sibling sizes to auto, addressing the “restore original size after expand” regression described in #17545.

Changes:

• Removed the initialization-time size reset that triggered when any panes were collapsed.
• Updated pane collapse handling to preserve size while clearing only temporary dragSize, and to allow adjacent fixed-size panes to flex-fill released space during a collapse.
• Added/updated unit tests covering horizontal/vertical collapse/expand, fixed-size siblings, initially-collapsed panes, and drag-size cleanup.

File summaries

File	Description
projects/igniteui-angular/splitter/src/splitter/splitter.component.ts	Stops resetting pane sizes during pane initialization when a pane starts collapsed.
projects/igniteui-angular/splitter/src/splitter/splitter.component.spec.ts	Adds coverage for preserving sizes/constraints across collapse/expand, including fixed-size and initially-collapsed scenarios.
projects/igniteui-angular/splitter/src/splitter/splitter-pane/splitter-pane.component.ts	Adjusts flex behavior to preserve configured sizes, clear only drag sizes on collapse changes, and allow fill behavior while a sibling is collapsed.

Review details

Suppressed comments (2)

projects/igniteui-angular/splitter/src/splitter/splitter.component.spec.ts:551

• Asserting exact pixel widths for all panes (toEqual([100, 100, 100])) may be brittle if splitter bars/borders/padding affect offsetWidth by a pixel. A tolerance-based assertion is typically more stable.

        expect(panes.map(pane => pane.element.offsetWidth)).toEqual([100, 100, 100]);

projects/igniteui-angular/splitter/src/splitter/splitter.component.spec.ts:319

• These strict offsetWidth equality assertions can be flaky due to rounding/layout differences after toggling collapse (even when the configured sizes are preserved). Consider asserting within a small pixel tolerance instead.

        expect(pane1.element.offsetWidth).toBe(pane1Width);
        expect(pane2.element.offsetWidth).toBe(pane2Width);

• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Lite

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