Gum Layout

Every element's position and size is a number plus a unit that says how to interpret the number relative to its parent, its children, or the screen. Getting the unit right — not the number — is what fixes almost all "it's in the wrong place / wrong size" problems. Docs: https://docs.flatredball.com/gum/code/layout/introduction-to-gum-layout.

The four position inputs

• X, Y — the numbers.
• XUnits, YUnits — how X/Y are measured. In C# these are GeneralUnitType (PixelsFromSmall = from the left/top edge, PixelsFromMiddle = from center, PixelsFromLarge = from the right/bottom edge, Percentage). (On disk these are stored as a different enum — see gum-file-format.)
• XOrigin, YOrigin — which point on the element is being placed (HorizontalAlignment Left/Center/Right, VerticalAlignment Top/Center/Bottom).

Example — center an element in its parent: set XOrigin/YOrigin to Center and XUnits/YUnits to PixelsFromMiddle with X = 0, Y = 0.

The two size inputs

• Width, Height — the numbers.
• WidthUnits, HeightUnits — DimensionUnitType. The ones you use most:

Unit	Meaning
Absolute	Value is pixels.
PercentageOfParent	Value is a % of the parent (100 = fill).
RelativeToParent	Parent size plus value px (0 = match parent, −20 = 20 px smaller).
RelativeToChildren	Sized to fit children, value = extra padding.
Ratio	Split leftover space among Ratio siblings.

Landmine: there is no WPF-style Auto

An element does not grow to fit its content unless you ask it to. A Button with Width = 128, WidthUnits = Absolute is 128 px wide no matter how long its text is. For content-sized elements use WidthUnits = RelativeToChildren.

Landmine: children-vs-parent size cycles

If a parent is RelativeToChildren and a child is PercentageOfParent on the same axis, the size is circular (parent waits for child, child waits for parent). Gum resolves it to avoid a crash, but the result is rarely what you want — make one side concrete.

Anchor & Dock (the easy path)

Prefer these over hand-setting units for common cases. They work on both Forms controls and raw visuals, and set several position/size properties at once:

• Anchor — snap to a spot: TopLeft, Top, Center, BottomRight, etc.
• Dock — attach and stretch: Fill, Top, Left, FillHorizontally, SizeToChildren, etc.

panel.Dock(Gum.Wireframe.Dock.Fill);   // fill the parent
title.Anchor(Gum.Wireframe.Anchor.Top); // pin to top-center

Docs: https://docs.flatredball.com/gum/code/layout/anchor-and-dock.

Stacking children

A container arranges its children by its ChildrenLayout:

• Regular — children positioned independently (the default).
• TopToBottomStack / LeftToRightStack — stack children in order; set StackSpacing for gaps, WrapsChildren = true to wrap into rows/columns.
• AutoGridHorizontal / AutoGridVertical — arrange into a uniform grid.

A StackPanel Forms control is a container pre-set to stack — vertical by default, with a Spacing property (the Forms name for StackSpacing). In code, add children to any container with AddChild (see gum-forms-controls); the ChildrenLayout then arranges them. Docs: https://docs.flatredball.com/gum/code/layout/stacking.

Quick recipes

Goal	Do
Fill parent	Dock(Dock.Fill) — or Width/Height = 0, units RelativeToParent
Center in parent	XOrigin/YOrigin = Center, XUnits/YUnits = PixelsFromMiddle, X/Y = 0
Size to text	WidthUnits/HeightUnits = RelativeToChildren
Vertical menu	container ChildrenLayout = TopToBottomStack, set StackSpacing

"Centered" is two different things. Centering the element in its parent is the recipe above (XOrigin/XUnits). A Text/Label's HorizontalAlignment only centers the text within the element's own width — if the element is auto-width, that looks left-aligned. Pick the one you actually mean.