Improve layout managers

ltml/SYNTAX.md

@@ -1169,6 +1169,15 @@ not automatically change paragraph shaping or bidi behavior inside text widgets.
 - Use `align-self="start"`, `align-self="center"`, or `align-self="end"` to control horizontal placement within the vbox row.
 - In `dir="rtl"`, `align-self="start"` means right and `align-self="end"` means left.
 - In `dir="rtl"`, children are flush against the right edge plus padding instead of the left.
+- When at least one child uses `height="auto"` and a height-constrained vbox
+  fragment has true surplus height beyond the specified, percent, and preferred
+  heights of the children on that fragment plus `layout.vpadding`, omitted
+  heights keep their preferred heights and the `auto` children split the
+  leftover height evenly.
+- In natural-height vboxes, and in constrained vboxes without surplus,
+  `height="auto"` behaves the same as an omitted height.
+- When a vbox splits across pages, `height="auto"` is evaluated separately for
+  each fragment page based only on the children present on that fragment.
 
 ### HBox Details
 
@@ -1178,6 +1187,12 @@ not automatically change paragraph shaping or bidi behavior inside text widgets.
 - Use `align-self="start"`, `align-self="center"`, or `align-self="end"` to control vertical placement within the hbox track.
 - In `hbox`, `align-self="start"` means top and `align-self="end"` means bottom.
 - Unaligned children share remaining width equally unless `width` is specified.
+- When at least one child uses `width="auto"` and the hbox has true surplus
+  width beyond the preferred widths of the remaining unsized children plus
+  hpadding, omitted widths keep their preferred widths and the `auto` children
+  split the leftover space evenly.
+- In constrained hboxes, and in layout managers without their own `auto`
+  sizing policy, `width="auto"` behaves the same as an omitted width.
 - In `dir="rtl"`, stacking order reverses: children flow right to left, `align="left"` pins to the right side, and `align="right"` pins to the left side.
 
 ### Table Details
@@ -1186,7 +1201,25 @@ not automatically change paragraph shaping or bidi behavior inside text widgets.
 - Set `rows` for column-major order (`order="cols"`).
 - Use `colspan` and `rowspan` attributes on cells to span multiple slots.
 - Column widths can be fixed (`width="120pt"`), percentage (`width="40%"`), or
-  automatic (equal share of remaining space).
+  omitted. Omitted columns keep the historical table behavior: they share the
+  remaining width equally.
+- When at least one single-column cell uses `width="auto"` and the table has
+  true surplus width beyond the preferred widths of omitted and auto columns,
+  omitted columns keep their preferred widths and auto columns split the
+  remaining width evenly. In constrained tables where omitted preferred widths
+  can still fit, omitted columns keep those preferred widths and auto columns
+  split the remaining width. Only when omitted preferred widths cannot fit does
+  the table fall back to equal sharing.
+- Cells with `colspan > 1` receive the resolved width of their spanned columns
+  but do not drive auto column sizing.
+- When at least one row contains a `height="auto"` cell and a height-constrained
+  table has true surplus height beyond fixed and preferred row heights, omitted
+  rows keep their preferred heights and auto rows split the remaining height
+  evenly.
+- In natural-height tables, and in constrained tables without surplus,
+  `height="auto"` behaves the same as an omitted height.
+- When a direct page-child table splits across pages, auto row height is
+  evaluated separately for each fragment page.
 - In `dir="rtl"`, columns are placed right to left (column 0 at the right edge).
 
 ### Flow Details
@@ -1353,6 +1386,7 @@ Measurements can be expressed in several forms:
 | With unit     | `1in`, `2.5cm`, `14pt` | Explicit unit overrides the current `units`. |
 | Percentage    | `50%`       | Percentage of the container's content width or height. |
 | Relative      | `+10`, `-5` | Offset from the container's content dimension. |
+| Auto          | `auto`      | Automatic layout-managed size. Supported by `hbox` width, `vbox` height, table column width, and table row height; elsewhere it behaves like omitting the dimension. |
 
 **Supported units:** `pt` (points), `in` (inches, 72pt), `cm` (centimeters, 28.35pt).
 

ltml/canvas_capture.go

@@ -51,6 +51,8 @@ func resetCanvasWidgetRenderState(root Widget) {
 		widget.SetPrinted(false)
 		widget.SetVisible(true)
 		widget.SetDisabled(false)
+		widget.ClearResolvedWidth()
+		widget.ClearResolvedHeight()
 		switch value := widget.(type) {
 		case *StdContainer:
 			value.activeChildren = nil

ltml/dimensions.go

@@ -7,21 +7,44 @@ import (
 	"fmt"
 	"regexp"
 	"strconv"
+	"strings"
+)
+
+type DimensionMode int8
+
+const (
+	DimUnspecified DimensionMode = iota
+	DimLiteral
+	DimPct
+	DimRel
+	DimAuto
 )
 
 type Dimensions struct {
-	sides     Sides
-	margin    Sides
-	padding   Sides
-	corners   Corners
-	width     float32
-	widthPct  float32
-	widthRel  float32
-	height    float32
-	heightPct float32
-	heightRel float32
-	widthSet  bool
-	heightSet bool
+	sides       Sides
+	margin      Sides
+	padding     Sides
+	corners     Corners
+	width       float32
+	height      float32
+	widthValue  float32
+	heightValue float32
+	widthMode   DimensionMode
+	heightMode  DimensionMode
+	widthValid  bool
+	heightValid bool
+}
+
+type dimensionState struct {
+	resolved float32
+	value    float32
+	mode     DimensionMode
+	valid    bool
+}
+
+type dimensionsState struct {
+	width  dimensionState
+	height dimensionState
 }
 
 var (
@@ -83,7 +106,10 @@ func (d *Dimensions) SetAttrs(attrs map[string]string, units Units) {
 	}
 
 	if width, ok := attrs["width"]; ok {
-		if rePct.MatchString(width) {
+		width = strings.TrimSpace(width)
+		if width == "auto" {
+			d.SetWidthAuto()
+		} else if rePct.MatchString(width) {
 			widthPct, _ := strconv.ParseFloat(width[:len(width)-1], 64)
 			d.SetWidthPct(widthPct)
 		} else if reRel.MatchString(width) {
@@ -95,7 +121,10 @@ func (d *Dimensions) SetAttrs(attrs map[string]string, units Units) {
 		}
 	}
 	if height, ok := attrs["height"]; ok {
-		if rePct.MatchString(height) {
+		height = strings.TrimSpace(height)
+		if height == "auto" {
+			d.SetHeightAuto()
+		} else if rePct.MatchString(height) {
 			heightPct, _ := strconv.ParseFloat(height[:len(height)-1], 64)
 			d.SetHeightPct(heightPct)
 		} else if reRel.MatchString(height) {
@@ -109,19 +138,64 @@ func (d *Dimensions) SetAttrs(attrs map[string]string, units Units) {
 }
 
 func (d *Dimensions) SetHeight(value float64) {
-	d.height, d.heightPct, d.heightRel, d.heightSet = float32(value), 0, 0, true
+	d.height = float32(value)
+	d.heightValue = float32(value)
+	d.heightMode = DimLiteral
+	d.heightValid = true
+}
+
+func (d *Dimensions) SetHeightAuto() {
+	d.height = 0
+	d.heightValue = 0
+	d.heightMode = DimAuto
+	d.heightValid = false
+}
+
+func (d *Dimensions) ClearHeight() {
+	d.height = 0
+	d.heightValue = 0
+	d.heightMode = DimUnspecified
+	d.heightValid = false
 }
 
 func (d *Dimensions) SetHeightPct(value float64) {
-	d.heightPct, d.height, d.heightRel, d.heightSet = float32(value), 0, 0, true
+	d.height = 0
+	d.heightValue = float32(value)
+	d.heightMode = DimPct
+	d.heightValid = false
 }
 
 func (d *Dimensions) SetHeightRel(value float64) {
-	d.heightRel, d.height, d.heightPct, d.heightSet = float32(value), 0, 0, true
+	d.height = 0
+	d.heightValue = float32(value)
+	d.heightMode = DimRel
+	d.heightValid = false
+}
+
+func (d *Dimensions) ResolveHeight(value float64) {
+	d.height = float32(value)
+	d.heightValid = true
+}
+
+func (d *Dimensions) ClearResolvedHeight() {
+	if d.heightMode == DimLiteral {
+		d.height = d.heightValue
+	} else {
+		d.height = 0
+	}
+	d.heightValid = false
 }
 
 func (d *Dimensions) HeightIsSet() bool {
-	return d.heightSet
+	if d.heightValid {
+		return true
+	}
+	switch d.heightMode {
+	case DimLiteral, DimPct, DimRel:
+		return true
+	default:
+		return false
+	}
 }
 
 func (d *Dimensions) SetTop(value float64) {
@@ -141,15 +215,52 @@ func (d *Dimensions) SetLeft(value float64) {
 }
 
 func (d *Dimensions) SetWidth(value float64) {
-	d.width, d.widthPct, d.widthRel, d.widthSet = float32(value), 0, 0, true
+	d.width = float32(value)
+	d.widthValue = float32(value)
+	d.widthMode = DimLiteral
+	d.widthValid = true
+}
+
+func (d *Dimensions) SetWidthAuto() {
+	d.width = 0
+	d.widthValue = 0
+	d.widthMode = DimAuto
+	d.widthValid = false
+}
+
+func (d *Dimensions) ClearWidth() {
+	d.width = 0
+	d.widthValue = 0
+	d.widthMode = DimUnspecified
+	d.widthValid = false
 }
 
 func (d *Dimensions) SetWidthPct(value float64) {
-	d.widthPct, d.widthRel, d.width, d.widthSet = float32(value), 0, 0, true
+	d.width = 0
+	d.widthValue = float32(value)
+	d.widthMode = DimPct
+	d.widthValid = false
 }
 
 func (d *Dimensions) SetWidthRel(value float64) {
-	d.widthRel, d.widthPct, d.width, d.widthSet = float32(value), 0, 0, true
+	d.width = 0
+	d.widthValue = float32(value)
+	d.widthMode = DimRel
+	d.widthValid = false
+}
+
+func (d *Dimensions) ResolveWidth(value float64) {
+	d.width = float32(value)
+	d.widthValid = true
+}
+
+func (d *Dimensions) ClearResolvedWidth() {
+	if d.widthMode == DimLiteral {
+		d.width = d.widthValue
+	} else {
+		d.width = 0
+	}
+	d.widthValid = false
 }
 
 func (d *Dimensions) String() string {
@@ -158,13 +269,57 @@ func (d *Dimensions) String() string {
 }
 
 func (d *Dimensions) WidthPctIsSet() bool {
-	return d.widthPct > 0
+	return d.widthMode == DimPct
 }
 
 func (d *Dimensions) WidthRelIsSet() bool {
-	return d.widthRel != 0
+	return d.widthMode == DimRel
 }
 
 func (d *Dimensions) WidthIsSet() bool {
-	return d.widthSet
+	if d.widthValid {
+		return true
+	}
+	switch d.widthMode {
+	case DimLiteral, DimPct, DimRel:
+		return true
+	default:
+		return false
+	}
+}
+
+func (d *Dimensions) WidthMode() DimensionMode {
+	return d.widthMode
+}
+
+func (d *Dimensions) HeightMode() DimensionMode {
+	return d.heightMode
+}
+
+func (d *Dimensions) SaveState() dimensionsState {
+	return dimensionsState{
+		width: dimensionState{
+			resolved: d.width,
+			value:    d.widthValue,
+			mode:     d.widthMode,
+			valid:    d.widthValid,
+		},
+		height: dimensionState{
+			resolved: d.height,
+			value:    d.heightValue,
+			mode:     d.heightMode,
+			valid:    d.heightValid,
+		},
+	}
+}
+
+func (d *Dimensions) RestoreState(state dimensionsState) {
+	d.width = state.width.resolved
+	d.widthValue = state.width.value
+	d.widthMode = state.width.mode
+	d.widthValid = state.width.valid
+	d.height = state.height.resolved
+	d.heightValue = state.height.value
+	d.heightMode = state.height.mode
+	d.heightValid = state.height.valid
 }