DocumentSheetBackdrop

DocumentSheetBackdrop

The DocumentSheetBackdrop draws the Document Sheet itself in Page View: the paper the Visual Document is laid out on, and the Page Break rule where each US Letter Page — turned by the Page Setup's Page Orientation (INV-061) — ends and the next begins. It is the visible half of the Sheet; how tall the Sheet is — always a whole number of Pages — is decided by the pure DocumentSheet rule and applied by PageView (INV-058).

• Class: UI.Controls.DocumentSheetBackdrop (derives from System.Windows.FrameworkElement)
• Placed by: MainWindow.xaml, in the editing surface's Grid before the MarkdownRichEditor, so it is drawn behind it.

Why behind the editor, not over it

A Page Break has to be a full-width rule across the Sheet — that is what makes a tall Sheet read as a stack of pages rather than one long strip. Drawn as an adorner (the pattern the CodeShadingAdorner and the ChangeHighlightAdorner use) it would be drawn over the Visual Document, and a break that lands on a line of prose would strike straight through it.

So the Sheet is drawn underneath instead: this element paints the paper and the rules, and Page View makes the editor's own Background transparent so they show through. A Page Break then passes under the text, the way a seam printed on paper would. Nothing else changes — the editor keeps its own border, caret, selection, and every adorner it already carries.

Note what this does not do: content flows continuously across a Page Break. The rule marks where a Page ends; it does not push the line that straddles it onto the next Page (INV-058).

How it works

• Sized to the editor, not the cell. Width and Height are bound to the editor's ActualWidth / ActualHeight, with HorizontalAlignment=Left and VerticalAlignment=Top. The surface Grid can be stretched taller than the Sheet by the canvas it sits in, so stretching to the cell would paint paper below the Sheet's own bottom edge.
• A rule per Page boundary. OnRender fills its whole area with SheetBrush, then draws a one-unit rule at each multiple of PageHeight below the bottom edge — half-pixel offset so it lands on a device pixel instead of blurring across two. The Sheet's own bottom is a page edge already, so it gets no rule. PageHeight is bound to the Page Setup's oriented page height (1056 units upright, 816 turned), so the rules follow the Page Orientation (INV-061).
• Recolour is free. Both brushes are resource references (EditorBackgroundBrush, PageBreakBrush) declared AffectsRender, so a theme switch repaints the Sheet and never reflows the document.
• Invisible outside Page View. Its Visibility is bound to WorkspaceViewModel.IsPageViewEnabled; with Page View off the editor paints its own background again and this element is collapsed.
• Never in the way. IsHitTestVisible is false, so clicks, selection, and drag all land on the editor behind — or rather, in front of — it.

Properties

Property	Type	Description
SheetBrush	Brush	The paper the Visual Document is laid out on. Defaults to the EditorBackgroundBrush of the active palette.
PageBreakBrush	Brush	The rule drawn where one Page ends and the next begins. Defaults to the PageBreakBrush of the active palette.
PageHeight	double	One Page's height in device-independent units — where the Page Break rules fall. Bind to WorkspaceViewModel.PageSetup.PageHeight so the rules follow the Page Orientation (INV-061). Defaults to the upright US Letter page (1056).

Usage

<!-- Before the editor in the same Grid cell, so it is drawn behind it. -->
<controls:DocumentSheetBackdrop Grid.Column="1"
                                HorizontalAlignment="Left" VerticalAlignment="Top"
                                Width="{Binding ActualWidth, ElementName=Editor}"
                                Height="{Binding ActualHeight, ElementName=Editor}"
                                PageHeight="{Binding PageSetup.PageHeight}"
                                Visibility="{Binding IsPageViewEnabled, Converter={StaticResource BooleanToVisibilityConverter}}" />
<controls:MarkdownRichEditor x:Name="Editor" Grid.Column="1" ... />