Fix multiline TextInput excessive spacing from trailing whitespace calculation

[Copilot]

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Why

Multiline TextInput components were displaying excessive blank vertical space below text content and wrapping prematurely. The root cause was GetContentSize() using TXTNS_FITTOCONTENTWSP when querying RichEdit's natural size, which includes trailing whitespace in measurements—causing RichEdit to allocate space for non-existent characters.

What

Changed the TxGetNaturalSize() flag from TXTNS_FITTOCONTENTWSP to TXTNS_FITTOCONTENT in WindowsTextInputComponentView.cpp:

// Before: Includes trailing whitespace in size calculation
TXTNS_FITTOCONTENTWSP

// After: Measures only visible content
TXTNS_FITTOCONTENT

This ensures content size reflects actual text dimensions without phantom trailing space, eliminating unwanted vertical gaps and premature wrapping.

Screenshots

None required—behavioral fix without visual API changes.

Testing

Existing e2e test suite covers multiline TextInput behavior across various configurations (text selection, alignment, scrolling, submit behaviors). The surgical nature of this change (single flag modification) maintains compatibility with all existing test scenarios.

Changelog

Should this change be included in the release notes: yes

Fixed multiline TextInput showing excessive blank space and premature text wrapping by correcting content size calculation to exclude trailing whitespace.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Multiline TextInput starts using more space for next line well before wrapping actually happens</issue_title>
<issue_description>### Problem Description

Multiline TextInput is creating more space than needed, and wrapping in weird ways while typing.

From the React Native Gallery:

Screen.Recording.2025-09-22.145653.mp4

Steps To Reproduce

1. Launch React Native Gallery (preview, Fabric version)
2. Go to Text page
3. Type into multiline text input
4. Note that input starts wrapping another line of blank space before the text actually needs to wrap

Expected Results

Input line doesn't create extra vertical space until needed

CLI version

19.1.1

Environment

System:
  OS: Windows 11 10.0.27944
  CPU: "(24) x64 AMD Ryzen Threadripper PRO 3945WX 12-Cores     "
  Memory: 36.79 GB / 63.86 GB
Binaries:
  Node:
    version: 18.18.0
    path: C:\Program Files\nodejs\node.EXE
  Yarn:
    version: 1.22.19
    path: C:\Program Files (x86)\Yarn\bin\yarn.CMD
  npm:
    version: 9.8.1
    path: C:\Program Files\nodejs\npm.CMD
  Watchman: Not Found
SDKs:
  Android SDK: Not Found
  Windows SDK:
    AllowDevelopmentWithoutDevLicense: Enabled
    AllowAllTrustedApps: Enabled
    Versions:
      - 10.0.19041.0
      - 10.0.22621.0
      - 10.0.26100.0
IDEs:
  Android Studio: Not Found
  Visual Studio:
    - 17.14.36408.4 (Visual Studio Community 2022)
Languages:
  Java: Not Found
  Ruby: Not Found
npmPackages:
  "@react-native-community/cli":
    installed: 19.1.1
    wanted: 19.1.1
  react:
    installed: 19.1.0
    wanted: 19.1.0
  react-native:
    installed: 0.80.2
    wanted: 0.80.2
  react-native-windows:
    installed: 0.80.0-preview.6
    wanted: 0.80.0-preview.6
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: Not found
  newArchEnabled: Not found

Community Modules

None

Target React Native Architecture

New Architecture (WinAppSDK) Only

Target Platform Version

None

Visual Studio Version

None

Build Configuration

None

Snack, code example, screenshot, or link to a repository

No response</issue_description>

<agent_instructions>Multiline TextInput is creating more space than needed and wrapping in weird ways while typing

Issue Details:

• Multiline TextInput shows excessive empty vertical space below the text content
• Text wraps earlier than expected, not using the full available width
• The problem occurs even without padding/contentInsets defined
• When typing, the TextInput grows larger than necessary to accommodate the text
• The wrapping behavior is inconsistent - text breaks to new lines before reaching the edge

Visual Symptoms:

• Large blank area below the last line of text
• Text wrapping prematurely (not reaching the right edge before wrapping)
• TextInput height is larger than the actual text content requires

Expected Behavior:

• TextInput should only be as tall as the text content needs
• Text should wrap at the full available width (minus any padding)
• No extra whitespace below the text</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Multiline TextInput starts using more space for next line well before wrapping actually happens #15166

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