Fix child boxes rendering on top of borders

Closes #4

internal/system/box.go

@@ -75,7 +75,8 @@ func (this *System) newBox (outer anyBox) *box {
 		if box.parent == nil { return nil }
 		parentCanvas := box.parent.getCanvas()
 		if parentCanvas == nil { return nil }
-		return parentCanvas.SubCanvas(box.bounds)
+		drawableArea := box.bounds.Intersect(box.parent.getInnerClippingBounds())
+		return parentCanvas.SubCanvas(drawableArea)
 	})
 	if outer == nil {
 		box.drawer = box
@@ -259,6 +260,24 @@ func (this *box) unsetAttr (kind tomo.AttrKind, user bool) {
 	}
 }
 
+func (this *box) unsetAllAttrs (user bool) {
+	// keep this in sync with tomo.AttrKind!
+	this.outer.unsetAttr(tomo.AttrKindColor, user)
+	this.outer.unsetAttr(tomo.AttrKindTexture, user)
+	this.outer.unsetAttr(tomo.AttrKindTextureMode, user)
+	this.outer.unsetAttr(tomo.AttrKindBorder, user)
+	this.outer.unsetAttr(tomo.AttrKindMinimumSize, user)
+	this.outer.unsetAttr(tomo.AttrKindPadding, user)
+	this.outer.unsetAttr(tomo.AttrKindGap, user)
+	this.outer.unsetAttr(tomo.AttrKindTextColor, user)
+	this.outer.unsetAttr(tomo.AttrKindDotColor, user)
+	this.outer.unsetAttr(tomo.AttrKindFace, user)
+	this.outer.unsetAttr(tomo.AttrKindWrap, user)
+	this.outer.unsetAttr(tomo.AttrKindAlign, user)
+	this.outer.unsetAttr(tomo.AttrKindOverflow, user)
+	this.outer.unsetAttr(tomo.AttrKindLayout, user)
+}
+
 func (this *box) setBounds (bounds image.Rectangle) {
 	if this.bounds == bounds { return }
 	this.bounds = bounds
@@ -624,6 +643,7 @@ func (this *box) recursiveReApply () {
 		// applicator for every box, it's so style applicators can cache
 		// information about the boxes they're linked to (like all rules
 		// with a matching role).
+		this.unsetAllAttrs(false)
 		this.lastStyleNonce = hierarchyStyleNonce
 		this.styleApplicator = hierarchy.newStyleApplicator()
 		this.invalidateStyle()

internal/system/containerbox.go

@@ -298,6 +298,10 @@ func (this *containerBox) getCanvas () canvas.Canvas {
 	return this.canvas.Value()
 }
 
+func (this *containerBox) getInnerClippingBounds () image.Rectangle {
+	return this.innerClippingBounds
+}
+
 func (this *containerBox) notifyMinimumSizeChange (child anyBox) {
 	this.invalidateMinimum()
 	size   := child.minimumSize()

internal/system/hierarchy.go

@@ -193,6 +193,10 @@ func (this *Hierarchy) getCanvas () canvas.Canvas {
 	return this.canvas
 }
 
+func (this *Hierarchy) getInnerClippingBounds () image.Rectangle {
+	return this.canvas.Bounds()
+}
+
 func (this *Hierarchy) getModifiers () input.Modifiers {
 	return this.modifiers
 }

internal/system/internal-iface.go

@@ -12,6 +12,9 @@ type parent interface {
 	getHierarchy            () *Hierarchy
 	// canvas returns the canvas held by the parent.
 	getCanvas               () canvas.Canvas
+	// getInnerClippingBounds returns the area of the canvas that children
+	// can draw to.
+	getInnerClippingBounds  () image.Rectangle
 	// notifyMinimumSizeChange informs the parent that the minimum size of
 	// one of its children has changed.
 	notifyMinimumSizeChange (anyBox)