diff --git a/docs/configuration.md b/docs/configuration.md
index fd026c94..43b8d73f 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -58,11 +58,11 @@ The viewport is the scrollable area whose position and size the scroller tracks;
 | Setting | Type | Default | Effect |
 | --- | --- | --- | --- |
 | `windowViewport` | `boolean` | `false` | Use the browser window for scrolling and viewport dimensions. |
-| `viewportElement` | `HTMLElement`, synchronous function returning one, or `null` | `null` | Use an explicit viewport instead of the content element's parent. Experimental. |
+| `viewportElement` | `HTMLElement`, `null`, or a synchronous function returning either | `null` | Use an explicit viewport instead of the content element's parent. Experimental. |
 | `horizontal` | `boolean` | `false` | Scroll and measure along the horizontal axis; the DOM/CSS layout must also be horizontal. |
 | `alignShortContentToEnd` | `boolean` | `false` | Align content shorter than the viewport to the bottom/right. |
 
-`windowViewport` takes precedence over `viewportElement`. A viewport factory is evaluated when the scroller is created; it must return an element synchronously. An invalid result falls back to the parent viewport.
+`windowViewport` takes precedence over `viewportElement`. A viewport factory is evaluated when the scroller is created; it must return an element or `null` synchronously. A `null` or invalid result falls back to the parent viewport.
 
 With `alignShortContentToEnd: true`, content shorter than the viewport is aligned to the bottom in vertical mode or to the right in horizontal mode; `false` aligns it to the top or left. For content that fills or exceeds the viewport, scrolling and the initial position selected by `startIndex` are unchanged. The setting neither reverses item order nor automatically keeps the viewport at the end when items are added.
 
diff --git a/src/classes/adapter.ts b/src/classes/adapter.ts
index 54befe8b..ff709ff5 100644
--- a/src/classes/adapter.ts
+++ b/src/classes/adapter.ts
@@ -442,7 +442,7 @@ export class Adapter<Item = unknown> implements IAdapter<Item> {
   }
 
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
-  reset(options?: IDatasourceOptional): any {
+  reset(options?: IDatasourceOptional<Item>): any {
     this.reloadCounter++;
     this.logger.logAdapterMethod('reset', options, ` of ${this.reloadId}`);
     this.workflow.call({
diff --git a/src/classes/item.ts b/src/classes/item.ts
index 54f9ad9d..216ff119 100644
--- a/src/classes/item.ts
+++ b/src/classes/item.ts
@@ -36,8 +36,8 @@ export class Item<Data = unknown> implements _Item<Data> {
     this.container.data = value;
   }
 
-  get element(): HTMLElement {
-    return this.container.element as HTMLElement;
+  get element(): HTMLElement | undefined {
+    return this.container.element;
   }
   set element(value: HTMLElement) {
     this.container.element = value;
@@ -68,7 +68,7 @@ export class Item<Data = unknown> implements _Item<Data> {
   }
 
   makeVisible(): void {
-    this.routines.makeElementVisible(this.element);
+    this.routines.makeElementVisible(this.element as HTMLElement);
     this.invisible = false;
   }
 
diff --git a/src/classes/settings.ts b/src/classes/settings.ts
index bb3bf826..d7f77dfa 100644
--- a/src/classes/settings.ts
+++ b/src/classes/settings.ts
@@ -26,7 +26,7 @@ export class Settings<Data = unknown> implements ISettings, IDevSettings {
   infinite: boolean;
   horizontal: boolean;
   windowViewport: boolean;
-  viewportElement: HTMLElement | (() => void) | null;
+  viewportElement: HTMLElement | (() => HTMLElement | null) | null;
   alignShortContentToEnd: boolean; // align content shorter than the viewport to the bottom/right
   onBeforeClip: ItemsProcessor | null; // if set, it will be run before clipping items from Buffer after they are hidden
   sizeStrategy: SizeStrategy; // "average" | "frequent", determines behavior of unknown items
diff --git a/src/classes/viewport.ts b/src/classes/viewport.ts
index f2733d06..26622579 100644
--- a/src/classes/viewport.ts
+++ b/src/classes/viewport.ts
@@ -96,7 +96,7 @@ export class Viewport {
       bwd ? i <= items.length - 1 : i >= 0;
       i += bwd ? 1 : -1
     ) {
-      const itemEdge = this.routines.getEdge(items[i].element, opposite);
+      const itemEdge = this.routines.getEdge(items[i].element as HTMLElement, opposite);
       diff = itemEdge - viewportEdge;
       if ((bwd && diff > 0) || (!bwd && diff < 0)) {
         item = items[i];
diff --git a/src/interfaces/adapter.ts b/src/interfaces/adapter.ts
index fee6441f..08f99aea 100644
--- a/src/interfaces/adapter.ts
+++ b/src/interfaces/adapter.ts
@@ -110,7 +110,6 @@ export interface AdapterInsertOptions<Data = unknown> {
   beforeIndex?: number;
   afterIndex?: number;
   decrease?: boolean;
-  virtualize?: boolean;
 }
 
 export interface AdapterReplaceOptions<Data = unknown> {
@@ -164,7 +163,7 @@ export interface IAdapter<Data = unknown> {
   readonly eof$: Reactive<boolean>;
   readonly paused: boolean;
   readonly paused$: Reactive<boolean>;
-  reset(datasource?: IDatasourceOptional): MethodResult;
+  reset(datasource?: IDatasourceOptional<Data>): MethodResult;
   reload(reloadIndex?: number | string): MethodResult;
   append(options: AdapterAppendOptions<Data>): MethodResult;
   append(items: Data | Data[], eof?: boolean): MethodResult; // old signature
diff --git a/src/interfaces/datasource.ts b/src/interfaces/datasource.ts
index 1e86b8be..e7650612 100644
--- a/src/interfaces/datasource.ts
+++ b/src/interfaces/datasource.ts
@@ -14,7 +14,7 @@ type DSGetCallback<T> = (
   success: SuccessFn<T>,
   fail?: ErrorFn
 ) => void;
-type DSGetObservable<T> = (index: number, count: number) => ObservableLike<T[]>;
+type DSGetObservable<T> = (index: number, count: number) => ObservableLike<T>;
 type DSGetPromise<T> = (index: number, count: number) => PromiseLike<T[]>;
 
 export type DatasourceGet<T> = DSGetCallback<T> | DSGetObservable<T> | DSGetPromise<T>;
diff --git a/src/interfaces/item.ts b/src/interfaces/item.ts
index dd1c4aee..bde21096 100644
--- a/src/interfaces/item.ts
+++ b/src/interfaces/item.ts
@@ -3,7 +3,7 @@ import { ItemAdapter } from './adapter';
 export interface Item<Data = unknown> {
   uid: number;
   $index: number;
-  element: HTMLElement;
+  element?: HTMLElement;
   data: Data;
   invisible: boolean;
   get: () => ItemAdapter<Data>;
diff --git a/src/interfaces/settings.ts b/src/interfaces/settings.ts
index 51d2ef19..4870f1ba 100644
--- a/src/interfaces/settings.ts
+++ b/src/interfaces/settings.ts
@@ -65,11 +65,11 @@ export interface Settings<T = unknown> {
   windowViewport?: boolean;
 
   /**
-   * Explicit viewport element, or a synchronous factory returning one; windowViewport takes precedence.
-   * An invalid factory result falls back to the content element's parent.
+   * Explicit viewport element, or a synchronous factory returning an element or null; windowViewport takes precedence.
+   * A null or invalid factory result falls back to the content element's parent.
    * Experimental. Default: null (the content element's parent).
    */
-  viewportElement?: HTMLElement | (() => void) | null;
+  viewportElement?: HTMLElement | (() => HTMLElement | null) | null;
 
   /**
    * Aligns content shorter than the viewport to the bottom vertically or to the right horizontally.
diff --git a/src/processes/adapter/insert.ts b/src/processes/adapter/insert.ts
index e7c54320..783c9eb1 100644
--- a/src/processes/adapter/insert.ts
+++ b/src/processes/adapter/insert.ts
@@ -5,6 +5,10 @@ import { Item } from '../../classes/item';
 import { Direction } from '../../inputs/index';
 import { AdapterInsertOptions, AdapterUpdateOptions } from '../../interfaces/index';
 
+interface InsertParams extends AdapterInsertOptions {
+  virtualize?: boolean;
+}
+
 export default class Insert extends BaseAdapterProcessFactory(AdapterProcess.insert) {
   static run(scroller: Scroller, options: AdapterInsertOptions): void {
     const { params } = Insert.parseInput(scroller, options);
@@ -19,7 +23,7 @@ export default class Insert extends BaseAdapterProcessFactory(AdapterProcess.ins
     });
   }
 
-  static doInsert(scroller: Scroller, params: AdapterInsertOptions): boolean {
+  static doInsert(scroller: Scroller, params: InsertParams): boolean {
     if (!Insert.insertEmpty(scroller, params)) {
       if (!Insert.insertInBuffer(scroller, params)) {
         if (!Insert.insertVirtually(scroller, params)) {
@@ -52,7 +56,7 @@ export default class Insert extends BaseAdapterProcessFactory(AdapterProcess.ins
     return true;
   }
 
-  static insertInBuffer(scroller: Scroller, params: AdapterInsertOptions): boolean {
+  static insertInBuffer(scroller: Scroller, params: InsertParams): boolean {
     const { before, after, beforeIndex, afterIndex, items, decrease } = params;
     const indexToInsert = scroller.buffer.getIndexToInsert(
       before || after,
diff --git a/tests/e2e/scaffolding/data.ts b/tests/e2e/scaffolding/data.ts
index 4dc7ee6a..0e842fd8 100644
--- a/tests/e2e/scaffolding/data.ts
+++ b/tests/e2e/scaffolding/data.ts
@@ -11,10 +11,10 @@ export const makeItems = (firstId: number, count: number): TestItem[] =>
 export const starGet = ((
   index: number,
   count: number,
-  success: (items: unknown[]) => void
+  success: (items: TestItem[]) => void
 ): void => {
   success(makeItems(index, count).map(item => ({ ...item, text: `${item.text} *` })));
-}) as DatasourceGet<unknown>;
+}) as DatasourceGet<TestItem>;
 
 const minSize = 1;
 const maxSize = 100;
diff --git a/tests/e2e/specs/adapter.reset-persistence.spec.ts b/tests/e2e/specs/adapter.reset-persistence.spec.ts
index 5b4f12c6..3802a71b 100644
--- a/tests/e2e/specs/adapter.reset-persistence.spec.ts
+++ b/tests/e2e/specs/adapter.reset-persistence.spec.ts
@@ -6,14 +6,14 @@ import {
   makeTest,
   Misc,
   starGet,
-  DatasourceGet,
   IDatasourceOptional,
-  TestConfig
+  TestConfig,
+  TestItem
 } from '../scaffolding';
 
 interface ResetVariant {
   isNew: boolean;
-  options: IDatasourceOptional;
+  options: IDatasourceOptional<TestItem>;
 }
 
 type PermanentProp = Adapter.id | Adapter.version;
@@ -63,10 +63,8 @@ const reset = (misc: Misc, variant: ResetVariant) => {
     return misc.adapter.reset(variant.options);
   }
 
-  const datasource = new Datasource<unknown>({
-    get:
-      variant.options.get ??
-      (misc.datasource.get as unknown as DatasourceGet<unknown>),
+  const datasource = new Datasource<TestItem>({
+    get: variant.options.get ?? misc.datasource.get,
     settings: variant.options.settings,
     devSettings: variant.options.devSettings
   });
diff --git a/tests/e2e/specs/adapter.reset.spec.ts b/tests/e2e/specs/adapter.reset.spec.ts
index 0e166dec..c68ce375 100644
--- a/tests/e2e/specs/adapter.reset.spec.ts
+++ b/tests/e2e/specs/adapter.reset.spec.ts
@@ -6,7 +6,8 @@ import {
   Routines,
   starGet,
   IDatasourceOptional,
-  TestConfig
+  TestConfig,
+  TestItem
 } from '../scaffolding';
 
 interface ResetSnapshot {
@@ -20,7 +21,7 @@ interface ResetSnapshot {
 
 interface ResetScenario {
   config: TestConfig;
-  options?: IDatasourceOptional;
+  options?: IDatasourceOptional<TestItem>;
   scrolls?: number;
   direction?: Direction;
   interruption?: boolean;
@@ -62,7 +63,7 @@ const settingsScenarios: ResetScenario[] = [baseConfigs[0], baseConfigs[2]].map(
       settings: {
         ...config.datasourceSettings,
         startIndex: 999
-      } as IDatasourceOptional['settings']
+      } as IDatasourceOptional<TestItem>['settings']
     }
   })
 );
@@ -172,7 +173,7 @@ const expectReset = (
   misc.expect.uniqueItems();
 };
 
-const reset = (misc: Misc, options?: IDatasourceOptional) =>
+const reset = (misc: Misc, options?: IDatasourceOptional<TestItem>) =>
   options ? misc.adapter.reset(options) : misc.adapter.reset();
 
 const expectScrollListeners = (
@@ -358,7 +359,7 @@ describe('Adapter Reset Spec', () => {
           const before = takeSnapshot(misc);
 
           const result = await misc.adapter.reset(
-            options as IDatasourceOptional
+            options as IDatasourceOptional<TestItem>
           );
 
           expect(result.success).toBe(false);
diff --git a/tests/unit/settings.spec.ts b/tests/unit/settings.spec.ts
index b61ba03c..6e5cfb69 100644
--- a/tests/unit/settings.spec.ts
+++ b/tests/unit/settings.spec.ts
@@ -210,7 +210,7 @@ describe('Settings', () => {
       expect(calls).toBe(1);
     });
 
-    it('falls back to null for invalid or asynchronous factory results', () => {
+    it('falls back to null when a factory does not return an element', () => {
       [
         undefined,
         null,