feat: Added virtual scroll component and sample implementation

.github/copilot-instructions.md

@@ -66,6 +66,10 @@ Here are the best practices and the style guide information.
 
 Here is a link to the most recent Angular style guide https://angular.dev/style-guide
 
+### CSS naming convention
+
+See [`css-naming-convention.md`](../css-naming-convention.md) for the component CSS class naming rules — CUBE CSS for new/actively-migrated components, BEM for the rest of the existing library.
+
 ### TypeScript Best Practices
 
 - Use strict type checking

CHANGELOG.md

@@ -13,8 +13,18 @@ All notable changes for each version of this project will be documented in this
 
 ## 22.2.0
 
+### General
+
+- The Excel style filtering search list, `IgxComboComponent` and `IgxSimpleComboComponent` are now virtualized by `IgxVirtualScrollComponent` instead of the `igxFor` directive. A row is measured in the DOM once it renders and the measured size replaces the estimate it started from; rows that have not rendered keep that estimate.
+    - The list markup changed accordingly: `igx-display-container` and the `igx-vhelper--vertical` scrollbar are replaced by the `igx-virtual-scroll` host and its `igx-vs__item` row wrappers. Applications and tests that reach into those elements directly need updating.
+    - `IgxComboComponent.virtualScrollContainer` and `IgxSimpleComboComponent.virtualScrollContainer` are marked `@hidden @internal`; their concrete type follows the engine the combo uses.
+    - `IgxDropDownComponent` accepts a content-projected `igx-virtual-scroll` in addition to `*igxFor`, which keeps working as documented. Selection and navigation behave the same either way.
+
 ### New Features
 
+- `IgxVirtualScrollComponent`
+    - Added `initialViewportSize`, the viewport size to render the first window against. A list that is hidden until the change detection pass that reveals it has no size to measure in that pass and would render nothing; this gives that first render a size to work from, and the host's own size takes over once it has been laid out.
+    - Added `dataWindow`, taking a loaded page of a larger collection as `{ items, startIndex, totalCount }`. The list is as long as `totalCount`, so the scrollbar spans the whole collection while only the page is in memory, and indices the page does not cover render nothing until a page that covers them arrives. `data` is unchanged and is used whenever `dataWindow` is not set.
 - `IgcFormControlDirective`
     - Added support for `igc-color-picker` so it can be bound with `ngModel` and `formControlName`, in the same way `igc-rating` is already supported.
 
@@ -38,6 +48,13 @@ All notable changes for each version of this project will be documented in this
 
 ### Bug Fixes
 
+- **Accessibility**
+    - Removed the nested list role from the internal virtual-scroll containers in Combo, Simple Combo and Excel-style filtering, preserving their existing listboxes and options.
+- `IgxDropDownComponent`
+    - Navigation and item lookup now use the same normalized `dataWindow` indices and total count as the projected virtual scroll, including fractional or non-finite metadata and pages extending past the declared total.
+- `IgxComboComponent`, `IgxSimpleComboComponent`
+    - Fixed remote pages changing position before their replacements arrive and redundant requests for an already loaded initial range. Changes to a positive `totalItemCount` refresh the list without rebinding data; a reduced total excludes out-of-range records before filtering and grouping.
+    - Reduced selection-resolution work during change detection. Each combo resolves its selection once per check and validates cached primitive-key matches before reusing them. Missing or invalid matches share one fallback scan; object keys retain deep-equality matching. In-place changes that create an earlier duplicate of a cached key are not detected without rebinding data.
 - `IgxCheckboxComponent`
     - Fixed the tick-mark icon rendering with the Indigo shape (rounded rect + custom path) inside CSS-scoped subtrees that use a different design system than the application's global theme, e.g. a `material`-themed widget nested inside an `indigo`-themed app. Both tick-mark variants are now always rendered and toggled purely via CSS (`@container style(--ig-theme: indigo)`), removing the dependency on JS-side theme detection that could go stale in nested/multi-theme scenarios (#15021).
 - **Ripple**

css-naming-convention.md

@@ -1,6 +1,26 @@
 # Ignite UI for Angular CSS Naming Convention
 
-Ignite UI for Angular uses the Two Dashes style of the [BEM naming convention](https://en.bem.info/methodology/naming-convention/) for CSS classes.
+## Going forward: CUBE CSS
+
+New and actively-migrated components follow a [CUBE CSS](https://cube.fyi)-inspired strategy instead of BEM. CUBE splits styles into four concerns:
+
+- **Composition** — layout-only rules that arrange a component's own internal structure (e.g. the wrapper elements a component renders around projected content). These stay component-scoped for now; the library does not yet have a shared, cross-component composition/layout utility layer.
+- **Utility** — single-purpose, reusable classes for one CSS concern at a time. Introduce these only where a genuine cross-component need exists; don't invent component-scoped "utility" classes as a substitute for Block/Exception naming.
+- **Block** — a component's own root class and any other independently addressable "thing" it renders (e.g. a directive-backed item wrapper). Blocks are flat class names: no `--`-style modifier chains. Where an element is purely an internal part of another block (not independently meaningful on its own), name it with the block as a prefix followed by `__` (e.g. `igx-virtual-scroll__track`) so its ownership is clear at a glance — this is a plain naming convention, not an invocation of BEM's element/modifier tooling (no `bem` Sass mixins, no `--` modifiers).
+- **Exception** — state and variants. Prefer `data-*` attributes reflecting a component's own input/state (e.g. `[data-orientation="horizontal"]`) over boolean modifier classes kept in sync by the component. Attributes are also naturally queryable and stylable without extra host-binding wiring.
+
+Rules for CUBE-based components:
+
+- Class names are lower-case, hyphen-separated words, prefixed with `igx-` (unchanged from before).
+- No `--` modifier classes for state that varies at runtime — use a `data-*` attribute instead.
+- No dependency on the `igniteui-theming/sass/bem` mixins (`b()`/`e()`/`m()`) — write plain selectors.
+- If the component has no design tokens (no color/typography/elevation surface of its own), it does not need a schema entry in `igniteui-theming` — only the structural `themes/_base.scss` file, wrapped in the shared `layer(base)` mixin so it still participates in the library's `ig.base` cascade layer.
+
+This does not retroactively apply to existing components — see below.
+
+## Existing components: BEM (Two Dashes style)
+
+The rest of the library (everything not explicitly migrated) uses the [BEM naming convention](https://en.bem.info/methodology/naming-convention/), Two Dashes style, generated via the `igniteui-theming/sass/bem` mixins (`b()`, `e()`, `m()`):
 
 Naming Convention:
 
@@ -12,4 +32,6 @@ ex.: `block-name__elem-name--mod-name`
  - Boolean modifiers are delimited by double hyphens (--).
  - Key-value type modifiers are not used.
 
-**Important!** Double hyphen within the comment (--) is perceived as part of the comment and therefore its presence lead to error during document validation. [HTML5 Specification](http://www.w3.org/TR/html5/syntax.html#comments)
+**Important!** Double hyphen within the comment (--) is perceived as part of the comment and therefore its presence lead to error during document validation. [HTML5 Specification](http://www.w3.org/TR/html5/syntax.html#comments)
+
+Existing BEM-based components are not required to migrate to CUBE. Only migrate a component's naming when you are already doing substantial, deliberate work on its styles — don't rename classes as a drive-by change.

projects/igniteui-angular/combo/README.md

@@ -50,6 +50,15 @@ public dataLoading(evt): void {
 What the combo exposes is a `virtualizationState` property that gives state of the combo - first index and the number of items that needs to be loaded.
 The service, should inform the combo for the total items that are on the server - using the `totalItemCount` property.
 
+The first rendered range does not emit `dataPreLoad` when the initial page already covers it.
+Later range changes still emit the event, so the consumer can cancel a superseded request.
+Capture the requested range when starting a fetch and cancel its subscription before replacing
+it; assigning `data` does not identify which request produced the page.
+
+Changing a positive `totalItemCount` refreshes the list without rebinding `data`. If the total
+shrinks, loaded records beyond it are excluded while the valid prefix keeps its position.
+This limit is applied before filtering and grouping; group headers are not remote records.
+
 
 ## Features
 
@@ -86,6 +95,16 @@ export class MyCombo {
 }
 ```
 
+With primitive value keys, resolving the selection reuses previously found records
+after verifying their index, identity and key. Missing or invalid matches share one
+fallback scan. Changing the data array, its length or `valueKey` clears the lookup;
+object keys retain deep-equality matching. A selected key whose record is not loaded
+is represented by `{ [valueKey]: key }` and is checked again on the next resolution.
+
+The hit validation does not detect a different record becoming an earlier duplicate
+of an already cached key without rebinding the data. This remains a known limitation
+of the lookup, not a guarantee of compatibility with every in-place mutation.
+
 ### Selection Events
 
 The `igx-combo` exposes both `selectionChanging` and `selectionChanged`.

projects/igniteui-angular/combo/src/combo/combo-dropdown.component.ts

@@ -33,14 +33,7 @@ export class IgxComboDropDownComponent extends IgxDropDownComponent implements I
 
     /** @hidden @internal */
     public override get scrollContainer(): HTMLElement {
-        // TODO: Update, use public API if possible:
-        return this.virtDir.dc.location.nativeElement;
-    }
-
-    protected get isScrolledToLast(): boolean {
-        const scrollTop = this.virtDir.scrollPosition;
-        const scrollHeight = this.virtDir.getScroll()!.scrollHeight;
-        return Math.floor(scrollTop + this.virtDir.igxForContainerSize) === scrollHeight;
+        return this.virtualization!.scrollElement;
     }
 
     protected get lastVisibleIndex(): number {
@@ -137,15 +130,19 @@ export class IgxComboDropDownComponent extends IgxDropDownComponent implements I
      * @hidden
      */
     public override navigateFirst() {
-        this.navigateItem(this.virtDir.igxForOf!.findIndex(e => !e?.isHeader));
+        // The first selectable entry can only be looked for in what is loaded. A page that
+        // starts further in does not hold it, so the collection's own start is the target.
+        this.navigateItem(this.virtualization?.startIndex === 0
+            ? this.virtualization.findIndex(e => !e?.isHeader)
+            : 0);
         this.combo.setActiveDescendant();
     }
 
     /**
      * @hidden
      */
     public override navigatePrev() {
-        if (this._focusedItem && this._focusedItem.index === 0 && this.virtDir.state.startIndex === 0) {
+        if (this._focusedItem && this._focusedItem.index === 0) {
             this.combo.focusSearchInput(false);
             this.focusedItem = null;
         } else {
@@ -159,7 +156,7 @@ export class IgxComboDropDownComponent extends IgxDropDownComponent implements I
      * @hidden
      */
     public override navigateNext() {
-        const lastIndex = this.combo.totalItemCount ? this.combo.totalItemCount - 1 : this.virtDir.igxForOf!.length - 1;
+        const lastIndex = (this.virtualization?.length ?? 0) - 1;
         if (this._focusedItem && this._focusedItem.index === lastIndex) {
             this.focusAddItemButton();
         } else {
@@ -185,7 +182,7 @@ export class IgxComboDropDownComponent extends IgxDropDownComponent implements I
      * @hidden @internal
      */
     public override updateScrollPosition() {
-        this.virtDir.getScroll()!.scrollTop = this._scrollPosition;
+        this.virtualization!.scrollPosition = this._scrollPosition;
     }
 
     /**
@@ -208,14 +205,15 @@ export class IgxComboDropDownComponent extends IgxDropDownComponent implements I
     }
 
     public override ngAfterViewInit() {
-        this.virtDir.getScroll()!.addEventListener('scroll', this.scrollHandler);
+        super.ngAfterViewInit();
+        this.scrollContainer.addEventListener('scroll', this.scrollHandler);
     }
 
     /**
      * @hidden @internal
      */
     public override ngOnDestroy(): void {
-        this.virtDir.getScroll()!.removeEventListener('scroll', this.scrollHandler);
+        this.virtualization?.scrollElement.removeEventListener('scroll', this.scrollHandler);
         super.ngOnDestroy();
     }