Fixes #4817. CharMap: handle zero-width ModifierSymbol runes without throwing

[Copilot]

Fixes #4817

CharMap.RenderGrapheme threw InvalidOperationException for UnicodeCategory.ModifierSymbol characters with column width 0 (e.g. emoji skin tone modifiers U+1F3FB–U+1F3FF), which are designed to combine with a preceding base glyph and cannot render standalone.

Changes

• CharMap.cs — RenderGrapheme: Add case UnicodeCategory.ModifierSymbol: rendering a highlighted "M" placeholder when width == 0 (consistent with Format → "F"), or the grapheme normally when width > 0
• CharMap.cs — default branch: Replace throw new InvalidOperationException with a highlighted "?" fallback, making the renderer resilient to any future zero-width Unicode categories not yet explicitly handled
• CharMapTests.cs: Add Draw_ModifierSymbol_CodePoint_Does_Not_Throw — draws a CharMap at U+1F3FB and asserts no exception

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

Original prompt

---

This section details on the original issue you should resolve

<issue_title>CharMap throws InvalidOperationException for zero-width ModifierSymbol runes (e.g. U+1F3FB skin tone modifiers)</issue_title>
<issue_description>## Bug Description

CharMap.RenderGrapheme throws an InvalidOperationException when rendering Unicode code points that have UnicodeCategory.ModifierSymbol and a column width of 0. This occurs when scrolling through or displaying emoji skin tone modifier characters (U+1F3FB-U+1F3FF).

Exception:

System.InvalidOperationException: The Rune has zero width and no special-case UnicodeCategory logic applies.

Root Cause

The RenderGrapheme local function in CharMap.OnDrawingContent has a switch on UnicodeCategory that handles several zero-width categories (Format, NonSpacingMark, SpacingCombiningMark, EnclosingMark, Control, etc.), but does not handle UnicodeCategory.ModifierSymbol.

Characters like U+1F3FB (EMOJI MODIFIER FITZPATRICK TYPE-1-2) are classified as ModifierSymbol and report a column width of 0 from GetColumns() because they are designed to combine with a preceding base emoji rather than render standalone. When these fall through to the default branch with width == 0, the exception is thrown.

Steps to Reproduce

1. Run the CharMap demo/view
2. Navigate to the U+1F3Fx range (emoji skin tone modifiers)
3. The view throws InvalidOperationException

Proposed Fix

Add a case UnicodeCategory.ModifierSymbol: to the switch statement in RenderGrapheme. Since these are modifier characters that cannot render standalone (zero width), they should display a highlighted placeholder (consistent with how Format characters show F):

// Modifier symbol (e.g. emoji skin tone modifiers U+1F3FB-U+1F3FF) that reports
// width of 0 because it is designed to combine with a preceding base emoji.
case UnicodeCategory.ModifierSymbol:
    if (width > 0)
    {
        AddStr (grapheme);
    }
    else
    {
        SetAttributeForRole (VisualRole.Highlight);
        AddStr ("M");
        SetAttributeForRole (VisualRole.Normal);
    }
    break;

Alternatively, the default branch could be made more resilient by replacing the throw with a fallback rendering, since other UnicodeCategory values with zero-width characters may exist or be introduced in future Unicode versions.

Affected File

• Terminal.Gui/Views/CharMap/CharMap.cs - RenderGrapheme local function inside OnDrawingContent</issue_description>

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

• Fixes CharMap throws InvalidOperationException for zero-width ModifierSymbol runes (e.g. U+1F3FB skin tone modifiers) #4817

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[Copilot]

Pull request overview

Fixes CharMap rendering crashes when encountering zero-width UnicodeCategory.ModifierSymbol runes (e.g., emoji skin tone modifiers), and makes the renderer resilient to any future zero-width categories by using a placeholder instead of throwing.

Changes:

• Add explicit rendering for UnicodeCategory.ModifierSymbol in CharMap.RenderGrapheme, showing a highlighted "M" placeholder when the rune’s column width is 0.
• Replace the zero-width default-case InvalidOperationException with a highlighted "?" fallback to prevent crashes.
• Add a unit test that draws CharMap at U+1F3FB and asserts no exception is thrown.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Terminal.Gui/Views/CharMap/CharMap.cs	Adds ModifierSymbol handling and replaces zero-width default-case throw with a safe placeholder render.
Tests/UnitTestsParallelizable/Views/CharMapTests.cs	Adds a draw regression test for a zero-width ModifierSymbol code point (U+1F3FB).

[BDisp]

Well handled.

[BDisp]

Cancelled after 60 minutes is a very long time. Are you sure it's running in degraded mode on Windows?

[tig]

The Link PR has a bug. I've fixed it in the kitty PR.