ContextQMD
Back to Terminal Gui

Wide Character Rendering Reproduction

Examples/WideCharRepro/README.md

2.4.12.3 KB
Original Source

Wide Character Rendering Reproduction

A minimal reproduction app demonstrating incorrect wide/fullwidth Unicode character rendering in terminal emulators.

The Problem

Terminal emulators must advance the cursor by 2 columns for "wide" Unicode characters (East Asian Width = Wide/Fullwidth, emoji with presentation selectors). Some terminals incorrectly advance by only 1 column, causing:

  • Grid misalignment / display "tearing"
  • Overlapping text
  • Broken TUI (text user interface) applications

Screenshots

Correct rendering (Windows Terminal)

All grid separators (│) are vertically aligned. Each wide character occupies exactly 2 terminal columns.

Broken rendering (affected terminals)

Grid separators are misaligned. Wide characters only advance the cursor by 1 column, causing all subsequent content on the line to shift left.

Running the Reproduction

bash
dotnet run

Requirements: .NET 10 SDK (or change TargetFramework in .csproj to your installed version, e.g., net8.0 or net9.0).

What to Look For

  1. Test 1–2 (Emoji / CJK grids): The │ separators should form perfectly vertical columns.
  2. Test 3 (Mixed content): Lines with mixed narrow + wide characters should fit within the box.
  3. Test 4 (Programmatic grid): Another grid alignment test with diverse wide characters.
  4. Test 5 (Column verification): The X markers should all appear at column 20.

If any of these are misaligned, the terminal has a wide-character width calculation bug.

Technical Details

Wide characters affected:

  • Emoji (U+1F600–U+1F64F, U+1F900–U+1F9FF, etc.)
  • CJK Unified Ideographs (U+4E00–U+9FFF)
  • CJK Compatibility Ideographs (U+F900–U+FAFF)
  • Fullwidth Forms (U+FF01–U+FF60)
  • Any character with East_Asian_Width property = W (Wide) or F (Fullwidth)

The terminal must use the Unicode East_Asian_Width property (or an equivalent wcwidth implementation) to determine cursor advancement after printing each character.