@syncfusion/ej2-spreadsheet/docs/skills/action/scroll.md

Feature-rich JavaScript Spreadsheet (Excel) control with built-in support for selection, editing, formatting, importing and exporting to Excel

# Scroll Module data flow

## What It Does
The Scroll module coordinates scrolling behavior across headers, main content, and virtualized viewports. Two-layer architecture: **Scroll (UI)** handles user input (wheel, pointer/touch, programmatic scroll events), computes offsets and visible ranges; **Workbook/Render (Core)** supplies row/column sizes and receives updated viewport indexes to render new content.

## Entry Points

**Scroll (UI Layer)**
- `constructor(parent)` / `addEventListener()` - initialize listeners and properties.
- `onContentScroll(e: ScrollEventArgs)` - central handler for horizontal/vertical scrolling updates.
- `getRowOffset(scrollTop, scrollDown)` / `getColOffset(scrollLeft, increase, skipHidden)` - map pixel scroll values to sheet row/column indexes and cumulative sizes.
- `updateTopLeftCell(increase, isLeft?)` - update the sheet's top-left cell based on offsets.
- `updateContainers()` - refresh floating containers like notes/comments on scroll.
- `setScrollEvent()` - attach DOM scroll/wheel handlers for headers and content.
- `onHeaderWheel(e)` / `onContentWheel(e)` - handle wheel events and apply horizontal/vertical scrolling (including Shift key behavior).
- `setClientX()`, `onTouchScroll()` / `pointerUpHandler()` - touch/pointer based scrolling support.
- `updateScrollValue(args)` - externally adjust internal offset bookkeeping for coordinated scrolls.
- `contentLoaded(args)` / `initProps()` / `destroy()` - lifecycle and cleanup.

**Workbook / Render (Core Layer)**
- `getRowHeight(sheet, idx, includeHidden?)` / `getColumnWidth(sheet, idx, skipHidden?, includeHidden?)` - provide per-row/col pixel sizes used by offset calculations.
- `skipHiddenIdx(sheet, index, fromTop, type?)` - map through hidden rows/cols when computing indexes.
- `getCellAddress(row, col)` - used when triggering refresh ranges for non-virtual expansion.
- `renderModule.refreshUI(...)` - invoked to materialize additional rows/columns when non-virtual thresholds are crossed.
- `sheetModule.getScrollSize()` - used for non-virtual scroll track sizing.
- Event notifications used/raised: `onHorizontalScroll`, `onVerticalScroll`, `updateNoteContainer`, `removeCommentContainer`, `focusRenameInput`, `virtualContentLoaded`, `colWidthChanged`.

## Core Logic Flow

```
Page load / contentLoaded() → initProps() and setScrollEvent()
    ↓
User scrolls (wheel/drag/touch) or programmatic scroll → onContentScroll(e)
    ↓
Compute left/top pixel positions → translate to offsets via getColOffset/getRowOffset
    ↓
updateTopLeftCell() → parent.updateTopLeftCell(...) updates active top-left indexes
    ↓
Notify systems: focusRenameInput, onHorizontalScroll/onVerticalScroll
    ↓
Update floating containers (notes/comments) and refresh UI if non-virtual thresholds reached
    ↓
Persist prevScroll / offset state for subsequent scroll deltas
```

## Operations Handled

1. Pixel-to-index translation: map `scrollLeft`/`scrollTop` to logical row/column indexes and cumulative sizes.
2. Virtualization-aware scrolling: compute when to request new row/column rendering and expand non-virtual dimensions when approaching end-of-sheet.
3. Wheel & header-wheel handling: prevent default behaviour as needed, support horizontal/vertical scrolls and Shift-key acceleration.
4. Touch/pointer scrolling: pointer/touch move handling with delta threshold and RAF-optimized scroll updates.
5. Container updates: notify note/comment containers and rename/focus handlers on scroll.
6. RTL support: initialize `initScrollValue` and translate `scrollLeft` accordingly for RTL scrolling.
7. External scroll syncing: `updateScrollValue` allows other modules to adjust internal offsets when their scroll content changes.

## Scroll Modes

| Mode | Behavior |
|------|----------|
| WheelScroll | Mouse-wheel driven vertical/horizontal scroll, with Shift modifier support. |
| ProgrammaticScroll | `updateScroll` / `onContentScroll` invoked by code to set scroll positions. |
| TouchScroll | Touch/pointer based horizontal scrolling using `pointermove`/`touchmove` with RAF updates. |
| VirtualScroll | Virtualized viewport calculations using `getRowOffset`/`getColOffset` to drive incremental rendering. |

## Validation & Safety

- **Virtualization bounds:** Respect `scrollSettings.enableVirtualization` and `isFinite` to avoid requesting rendering beyond allowed limits.
- **Hidden rows/cols:** Use `skipHiddenIdx`/`getColumnWidth` with `skipHidden` flags to ensure offsets map correctly around hidden items.
- **Small scroll jitter:** Compare rounded values and allow 1px tolerance to account for rendering differences across devices/resolutions.
- **RTL adjustments:** Normalize `scrollLeft` to `initScrollValue` when RTL is enabled.
- **Prevent default where needed:** Wheel handlers call `preventDefault()` when appropriate to avoid native browser overscroll.
- **Touch filtering:** Ignore pointer events originated from mouse in touch handlers; use `selectionStatus` to detect overlay/touch selection interactions.

## Desired Outputs

**User-Facing:**
- Smooth, responsive scrolling across headers and main content.
- Correct cursor/wheel behavior with Shift and touch interactions.
- Floating note/comment containers and rename inputs remain correctly positioned during scroll.
- Seamless virtualization experience: content loads as the user approaches the viewport edge.

**System-Level:**
- Accurate `offset` bookkeeping: `offset.left` and `offset.top` store `{ idx, size }` for current viewport.
- Notifications emitted: `onHorizontalScroll`, `onVerticalScroll`, `updateNoteContainer`, `removeCommentContainer`, `focusRenameInput`.
- Triggered render refreshes when non-virtual thresholds are crossed (via `renderModule.refreshUI`).
- Internal `prevScroll` updated to enable delta calculations and avoid redundant work.
- Compatibility with RTL, high-DPI, and mixed input (mouse/touch) environments.