Bug Report: Wide/Fullwidth Character Rendering Causes Display Tearing

Summary

The terminal incorrectly renders wide (fullwidth) Unicode characters, advancing the cursor by only 1 column instead of the required 2. This causes all subsequent content on the line to shift left, breaking grid alignment and causing display "tearing" in TUI applications.

Reproduction

git clone https://github.com/gui-cs/Terminal.Gui.git
cd Terminal.Gui/Examples/WideCharRepro
dotnet run

Or paste this minimal C# program (requires .NET 8+):

using System.Text;

Console.OutputEncoding = Encoding.UTF8;

Console.WriteLine ("TEST: Emoji grid (each emoji = 2 columns)");
Console.WriteLine ("┌──┬──┬──┬──┬──┬──┬──┬──┐");
Console.WriteLine ("│😀│😁│😂│🤣│😄│😅│😆│😇│");
Console.WriteLine ("│──│──│──│──│──│──│──│──│");
Console.WriteLine ("│🐶│🐱│🐭│🐹│🐰│🦊│🐻│🐼│");
Console.WriteLine ("└──┴──┴──┴──┴──┴──┴──┴──┘");
Console.WriteLine ();
Console.WriteLine ("TEST: Column alignment (X should be at column 20)");
Console.WriteLine ("01234567890123456789X  <- 20 narrow (20×1=20)");
Console.WriteLine ("😀😁😂😃😄😅😆😇😈😉X  <- 10 emoji  (10×2=20)");
Console.WriteLine ("你好世界测试宽字符验X  <- 10 CJK    (10×2=20)");

Expected Behavior

Each wide character (emoji, CJK ideograph, fullwidth form) should advance the cursor by 2 columns. Grid separators (│) should be vertically aligned:

┌──┬──┬──┬──┬──┬──┬──┬──┐
│😀│😁│😂│🤣│😄│😅│😆│😇│
│──│──│──│──│──│──│──│──│
│🐶│🐱│🐭│🐹│🐰│🦊│🐻│🐼│
└──┴──┴──┴──┴──┴──┴──┴──┘

The X markers should vertically align at column 20:

01234567890123456789X
😀😁😂😃😄😅😆😇😈😉X
你好世界测试宽字符验X

Actual Behavior

Wide characters advance the cursor by only 1 column. Grids misalign and text overlaps:

┌──┬──┬──┬──┬──┬──┬──┬──┐
│😀│😁│😂│🤣│😄│😅│😆│😇│   <- shifted left
│──│──│──│──│──│──│──│──│
│🐶│🐱│🐭│🐹│🐰│🦊│🐻│🐼│   <- shifted left
└──┴──┴──┴──┴──┴──┴──┴──┘

Terminal Compatibility Matrix

To contributors: please fill in your terminal's results and submit a PR or comment.

Affected Unicode Ranges

• 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 codepoint with Unicode East_Asian_Width = W (Wide) or F (Fullwidth)

Impact

This bug breaks any TUI application that renders wide characters in a grid or alongside narrow characters. Affected applications include:

• Terminal.Gui (cross-platform .NET TUI toolkit)
• Any ncurses/curses-based app using CJK or emoji
• tmux, vim, etc. when displaying wide characters

Relevant Standards

• Unicode TR#11 – East Asian Width
• POSIX wcwidth(3)
• Unicode TR#51 – Emoji Presentation

Environment

• OS: [e.g., Windows 11 24H2]
• Terminal: [e.g., VS Code Insiders 1.x integrated terminal]
• Shell: [e.g., PowerShell 7.5]
• Font: [e.g., Cascadia Code NF]
• Locale: [e.g., en-US, UTF-8]

Screenshots