I am making progress with the tree view morph.

package-sources/Bootstrap.EnvironmentSetup/Function.sysmel

@@ -92,4 +92,13 @@ Function
     withSelector: #makeNoGC addMethod: {:(Function)self :: Function |
         self addFlags: FunctionFlags::NoGC.
         self
+    } withEagerAnalysis;
+    withSelector: #validPrimitiveName addMethod: {:(Function)self :: Symbol |
+        if: (self __slotNamedAt__: #captureEnvironment) == __pendingMemoizationValue__ then: {
+            let: #definition type: AnyValue with: (self __slotNamedAt__: #definition).
+            definition primitiveName
+        } else: {
+            self ensureAnalysis.
+            self __slotNamedAt__: #primitiveName
+        }.
     } withEagerAnalysis.

package-sources/Core.Runtime/BaseBootstrap/AST.sysmel

@@ -254,8 +254,8 @@ ASTLiteralNode
             else: false
     } makeOverride;
     withSelector: #literalFunctionPrimitiveName addMethod: {:(ASTLiteralNode)self :: Symbol |
-        if: (self value __type__ lookupSelector: #primitiveName) ~~ nil
-            then: (self value ensureAnalysis; primitiveName)
+        if: (self value __type__ lookupSelector: #validPrimitiveName) ~~ nil
+            then: (self value validPrimitiveName)
             else: nil
     } makeOverride.
 

package-sources/Graphics.Font/FontFace.sysmel

@@ -15,9 +15,15 @@ public abstract class FontFace definition: {
     public virtual method measureWidthForString: (string: SequenceableCollection) from: (startIndex: Size) until: (endIndex: Size) ::=> Float32
         := startIndex < endIndex ifTrue: (endIndex - startIndex) asFloat32 * self width ifFalse: 0f32.
 
+    public virtual method measureWidthForString: (string: SequenceableCollection) ::=> Float32
+        := self measureWidthForString: string from: 0sz until: string size.
+
     public virtual method measureExtentForString: (string: SequenceableCollection) from: (startIndex: Size) until: (endIndex: Size) ::=> Float32x2
         := (self measureWidthForString: string from: startIndex until: endIndex) @ self height.
 
+    public virtual method measureExtentForString: (string: SequenceableCollection) ::=> Float32x2
+        := self measureExtentForString: string from: 0sz until: string size.
+
     public virtual method measureBoundsForString: (string: SequenceableCollection) from: (startIndex: Size) until: (endIndex: Size) ::=> RectangleF32
         := 0f32x2 extent: (self measureExtentForString: string from: startIndex until: endIndex).
 }.

package-sources/Morphic.Core/BasicDevelopmentEnvironment.sysmel

@@ -0,0 +1,26 @@
+public class BasicDevelopmentEnvironment superclass: PanelMorph; definition: {
+    public override method initialize => Void := {
+        super initialize.
+
+        let workspaceButton := SimpleButtonMorph new
+            label: "Workspace";
+            when: MorphActivatedAnnouncement do: {:: Void | Workspace new openInWindowSystem};
+            yourself.
+
+        let systemBrowserButton := SimpleButtonMorph new
+            label: "System Browser";
+            when: MorphActivatedAnnouncement do: {:: Void | SystemBrowser new openInWindowSystem};
+            yourself.
+
+        self
+            addMorph: workspaceButton;
+            addMorph: systemBrowserButton;
+            layout: (MorphVerticalBoxLayout new
+                add: workspaceButton;
+                add: systemBrowserButton;
+                yourself
+            )
+    }.
+
+    public override method defaultExtent => Float32x2 := 150f32@100f32.
+}.

package-sources/Morphic.Core/BasicInspector.sysmel

@@ -26,7 +26,7 @@ public class BasicInspectorListElement definition: {
     }
 }.
 
-public class BasicInspector superclass: BorderedMorph; definition: {
+public class BasicInspector superclass: PanelMorph; definition: {
     protected field inspectedValue => Untyped.
     protected field listView => ListViewMorph.
     protected field sourceCodeEditor => SourceCodeEditorMorph.
@@ -53,8 +53,6 @@ public class BasicInspector superclass: BorderedMorph; definition: {
             )
     }.
 
-    public override method defaultColor => Color := Color transparent.
-
     public override method defaultExtent => Float32x2 := 300f32@300f32.
 
     public method onActivatedListViewElement: (element: BasicInspectorListElement) ::=> Void := {

package-sources/Morphic.Core/DataSource.sysmel

@@ -1,7 +1,21 @@
 public abstract class DataSource definition: {
+    public field onUpdate => (() => Void).
+
     public abstract method numberOfElements => Size
         := self subclassResponsibility.
 
+    public abstract method elementModelAtRow: (rowIndex: Size) ::=> AnyValue
+        := self subclassResponsibility.
+
     public abstract method elementAtRow: (rowIndex: Size) ::=> AnyValue
         := self subclassResponsibility.
+
+    public virtual method depthOfRow: (rowIndex: Size) ::=> UInt32
+        := 0u32.
+
+    public method notifyUpdates => Void := {
+        onUpdate ifNotNil: {
+            onUpdate()
+        }
+    }.
 }.

package-sources/Morphic.Core/PanelMorph.sysmel

@@ -0,0 +1,3 @@
+public class PanelMorph superclass: BorderedMorph; definition: {
+    public override method defaultColor => Color := Color transparent.
+}.

package-sources/Morphic.Core/SimpleListDataSource.sysmel

@@ -9,6 +9,11 @@ public class SimpleListDataSource superclass: DataSource; definition: {
     public override method numberOfElements => Size
         := items size.
 
+    public override method elementModelAtRow: (rowIndex: Size) ::=> AnyValue
+        := rowIndex < items size
+            ifTrue: (items at: rowIndex)
+            ifFalse: nil.
+
     public override method elementAtRow: (rowIndex: Size) ::=> AnyValue
         := rowIndex < items size
             ifTrue: (items at: rowIndex)

package-sources/Morphic.Core/SimpleTreeDataSource.sysmel

@@ -0,0 +1,116 @@
+public class SimpleTreeDataSource superclass: DataSource.
+
+public class SimpleTreeDataSourceElement definition: {
+    public field depth => UInt32.
+    public field dataSource => SimpleTreeDataSource.
+    public field value => AnyValue.
+    public field isExpanded => Boolean.
+    public field children => OrderedCollection.
+
+    public method validExpandedChildrenWithBlock: (childrenBlock: (AnyValue) => Collection) ::=> OrderedCollection := {
+        children ifNotNil: {return: children}.
+
+        children := OrderedCollection new.
+        let childrenCollection := childrenBlock(value).
+        childrenCollection ifNotNil: {
+            childrenCollection do: {:eachChild :: Void |
+                children add: (SimpleTreeDataSourceElement new
+                    depth: depth + 1u32;
+                    dataSource: dataSource;
+                    value: eachChild;
+                    yourself)
+            }
+        }.
+
+        children
+    }.
+
+    public method expand => Void := {
+        isExpanded ifTrue: {return: void}.
+
+        isExpanded := true.
+        dataSource elementExpansionChanged: self.
+    }.
+
+    public method collapse => Void := {
+        isExpanded ifFalse: {return: void}.
+
+        isExpanded := false.
+        dataSource elementExpansionChanged: self.
+    }.
+
+    public method toggleExpansion => Void := {
+        isExpanded := isExpanded not.
+        dataSource elementExpansionChanged: self.
+    }.
+
+    public method hasChildren => Boolean
+        := children isNil || children isNotEmpty.
+}.
+
+SimpleTreeDataSource definition: {
+    let ChildrenBlock := (AnyValue) => Collection.
+
+    protected field roots => OrderedCollection.
+    protected field expandedItems => OrderedCollection.
+    public field childrenBlock => ChildrenBlock.
+
+    public override method initialize => Void := {
+        super initialize.
+        roots := OrderedCollection new.
+        expandedItems := OrderedCollection new.
+    }.
+
+    public override method numberOfElements => Size
+        := expandedItems size.
+
+    public override method elementModelAtRow: (rowIndex: Size) ::=> AnyValue
+        := rowIndex < expandedItems size
+            ifTrue: (expandedItems at: rowIndex)
+            ifFalse: nil.
+
+    public override method elementAtRow: (rowIndex: Size) ::=> AnyValue
+        := rowIndex < expandedItems size
+            ifTrue: (expandedItems at: rowIndex) value
+            ifFalse: nil.
+
+    public override method depthOfRow: (rowIndex: Size) ::=> UInt32
+        := rowIndex < expandedItems size
+            ifTrue: (expandedItems at: rowIndex) depth
+            ifFalse: 0u32.
+
+    public method roots: (newRoots: Collection) ::=> Void := {
+        roots := OrderedCollection new.
+        newRoots do: {:element :: Void |
+            roots add: (SimpleTreeDataSourceElement new
+                depth: 0u32;
+                dataSource: self;
+                value: element;
+                yourself)
+        }.
+
+        self recomputeExpandedElements.
+    }.
+
+    public method recomputeExpandedElements => Void := {
+        expandedItems := OrderedCollection new.
+        roots do: {:(SimpleTreeDataSourceElement)each :: Void |
+            self addElementWithExpandedChildrenFrom: each
+        }.
+
+        self notifyUpdates
+    }.
+
+    public method elementExpansionChanged: (element: SimpleTreeDataSourceElement) ::=> Void := {
+        self recomputeExpandedElements
+    }.
+
+    public method addElementWithExpandedChildrenFrom: (element: SimpleTreeDataSourceElement) ::=> Void := {
+        expandedItems add: element.
+        element isExpanded ifFalse: {return: void}.
+
+        (element validExpandedChildrenWithBlock: childrenBlock) do: {:(SimpleTreeDataSourceElement)eachChild :: Void |
+            self addElementWithExpandedChildrenFrom: eachChild
+        }
+    }.
+}.

package-sources/Morphic.Core/StringTableViewColumn.sysmel

@@ -5,13 +5,14 @@ public class StringTableViewColumn superclass: TableViewColumn; definition: {
 
     public override method initialize => Void := {
         super initialize.
+        proportion := 1f32.
         display := {:element :: String | element asString }.
     }.
 
-    public override method createCellMorphForElement: element ::=> Morph
+    public override method createCellMorphForElement: element model: elementModel ::=> Morph
         := display(element) asMorph.
 
-    public override method updateCellMorph: (morph: Morph) withElement: element ::=> Void := {
+    public override method updateCellMorph: (morph: Morph) withElement: element model: elementModel ::=> Void := {
         (morph downCastTo: StringMorph) contents: display(element)
     }.
 }.

package-sources/Morphic.Core/SystemBrowser.sysmel

@@ -0,0 +1,9 @@
+public class SystemBrowser superclass: PanelMorph; definition: {
+    protected field sourceCodeEditor => SourceCodeEditorMorph.
+
+    public override method initialize => Void := {
+        super initialize.
+    }.
+
+    public override method defaultExtent => Float32x2 := 600f32@400f32.
+}.

package-sources/Morphic.Core/TableViewColumn.sysmel

@@ -1,6 +1,7 @@
 public abstract class TableViewColumn definition: {
     public field name => String.
     public field width => Float32.
+    public field proportion => Float32.
 
     __Meta__ definition: {
         public method named: (name: String) ::=> self
@@ -10,10 +11,10 @@ public abstract class TableViewColumn definition: {
     public virtual method minimalCellExtent ::=> Float32x2
         := 0f32 @ FontFace default height.
 
-    public virtual method createCellMorphForElement: element ::=> Morph
+    public virtual method createCellMorphForElement: element model: elementModel ::=> Morph
         := Morph new.
 
-    public virtual method updateCellMorph: (morph: Morph) withElement: element ::=> Void := {
+    public virtual method updateCellMorph: (morph: Morph) withElement: element model: elementModel ::=> Void := {
         ## By default do nothing.
     }.
 }.

package-sources/Morphic.Core/TableViewMorph.sysmel

@@ -55,18 +55,42 @@ public class TableViewMorph superclass: BorderedMorph; definition: {
 
     public method dataSource: (newDataSource: DataSource) ::=> Void := {
         dataSource := newDataSource.
+        dataSource onUpdate: {:: Void |
+            self onDataUpdate
+        }.
         self changed.
     }.
 
     public method isRowSelected: (rowIndex: Size) ::=> Boolean
         := selectedRows isNotNil && (selectedRows includesRowIndex: rowIndex).
 
+    public method onDataUpdate => Void := {
+        contentMorph updateContent.
+        self changed
+    }.
+
     public method computeNewColumnWidths => Void := {
         columns isEmpty ifTrue: {return: void}.
 
-        let columnFixedWidth := self extent x / columns size asFloat32.
+        let totalProportion mutable := 0f32.
+        let fixedWidth mutable := 0f32.
+        columns do: {:(TableViewColumn)eachColumn :: Void |
+            let columnProportion := eachColumn proportion.
+            columnProportion = 0f32 ifTrue: {
+                fixedWidth := fixedWidth + eachColumn minimalCellExtent x
+            } ifFalse: {
+                totalProportion := totalProportion + columnProportion
+            }.
+        }.
+
+        let availableWidth := self extent x - fixedWidth max: 0f32.
         columns do: {:(TableViewColumn)eachColumn :: Void |
-            eachColumn width: columnFixedWidth
+            let columnProportion := eachColumn proportion.
+            columnProportion = 0f32 ifTrue: {
+                eachColumn width: eachColumn minimalCellExtent x
+            } ifFalse: {
+                eachColumn width: (availableWidth * columnProportion / totalProportion) floor
+            }.
         }
     }.
 

package-sources/Morphic.Core/TableViewRowMorph.sysmel

@@ -30,8 +30,9 @@ TableViewRowMorph definition: {
         dataSource ifNil: {return: void}.
 
         let element := dataSource elementAtRow: rowIndex.
+        let elementModel := dataSource elementModelAtRow: rowIndex.
         cells := self columns collect: {:(TableViewColumn)eachColumn :: Morph |
-            let cellMorph := eachColumn createCellMorphForElement: element.
+            let cellMorph := eachColumn createCellMorphForElement: element model: elementModel.
             self addMorph: cellMorph.
             cellMorph
         } as: Array.
@@ -43,14 +44,18 @@ TableViewRowMorph definition: {
 
         let columns := self columns.
         let element := dataSource elementAtRow: rowIndex.
+        let elementModel := dataSource elementModelAtRow: rowIndex.
         cells doWithIndex: {:(Morph)cellMorph :(Size)cellIndex :: Void |
             let viewColumn => TableViewColumn := columns at: cellIndex.
-            viewColumn updateCellMorph: cellMorph withElement: element
+            viewColumn updateCellMorph: cellMorph withElement: element model: elementModel
         }.
     }.
 
     public override method updateLayout => Void := {
-        let cellX mutable := 0f32.
+        let rowDepth := self dataSource depthOfRow: rowIndex.
+        let spaceWidth := FontFace default measureWidthForString: " ".
+
+        let cellX mutable := rowDepth asFloat32 * spaceWidth.
         let cellHeight := self extent y.
 
         let columns := self columns.