SourceGutter

SourceGutter

The Source Gutter: the presentation-only margin strip drawn to the left of the Source Panel's TextBox. It shows a Line Number for each visible source line of the Markdown Document, mirroring the Source Panel's own line layout and scroll position.

• Class: UI.Controls.SourceGutter (derives from System.Windows.Controls.Canvas)
• Default style: src/UI/Controls/SourceGutter.xaml (merged in App.xaml)

Authored as a custom Control (class plus a ResourceDictionary for its look), per the project's Control exception to the zero-code-behind rule. The gutter is view-only: it reads the Source Panel's line layout and mirrors it, and never mutates the document — so it cannot change the Markdown source (INV-014). It is the Source Panel counterpart of the EditorGutter, which serves the Visual Document.

How it works

The gutter positions its numbers from TextBox.GetRectFromCharacterIndex, which reports each line's position relative to the Source Panel's viewport. It re-lays out whenever the Source Panel's text, size, or scroll offset changes (via TextChanged, SizeChanged, and the bubbled ScrollViewer.ScrollChangedEvent), so the numbers stay aligned as the user types and scrolls.

The Source Panel is set to TextWrapping="NoWrap", so each source line occupies exactly one rendered row. A Line Number here is therefore the 1-based ordinal of a source line. Only the lines currently within the viewport (GetFirstVisibleLineIndex … GetLastVisibleLineIndex) are walked and drawn.

Properties

Property	Type	Description
Source	TextBox?	The Source Panel TextBox whose source lines this gutter mirrors. Setting it wires the refresh subscriptions.
LineNumberBrush	Brush	Brush for Line Numbers. Themed to MutedTextBrush.

Usage

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition Width="*" />
    </Grid.ColumnDefinitions>
    <controls:SourceGutter Grid.Column="0" Source="{Binding ElementName=SourcePanel}" />
    <TextBox Grid.Column="1" x:Name="SourcePanel" TextWrapping="NoWrap"
             Text="{Binding ActiveSession.Markdown, UpdateSourceTrigger=PropertyChanged, Delay=120}" />
</Grid>