Use a more sophisticated algorithm to distribute widget heights.

This is needed since there can now be an arbitrary number of widgets competing
for vertical space.

- Add a new MaxHeight method to the tk.Widget interface to provide hint on how
  to distribute the height.

- Implement the algorithm in distributeHeight in cli/app.go.

pkg/cli/app.go

@@ -4,6 +4,7 @@ package cli
 import (
 	"io"
 	"os"
+	"sort"
 	"sync"
 	"syscall"
 
@@ -259,7 +260,7 @@ func (a *app) redraw(flag redrawFlag) {
 		if hideRPrompt {
 			a.codeArea.MutateState(func(s *tk.CodeAreaState) { s.HideRPrompt = true })
 		}
-		bufMain := renderApp(a.codeArea, nil /* addon */, width, height)
+		bufMain := renderApp([]tk.Widget{a.codeArea /* no addon */}, width, height)
 		if hideRPrompt {
 			a.codeArea.MutateState(func(s *tk.CodeAreaState) { s.HideRPrompt = false })
 		}
@@ -269,7 +270,7 @@ func (a *app) redraw(flag redrawFlag) {
 		a.TTY.UpdateBuffer(bufNotes, bufMain, flag&fullRedraw != 0)
 		a.TTY.ResetBuffer()
 	} else {
-		bufMain := renderApp(a.codeArea, addons, width, height)
+		bufMain := renderApp(append([]tk.Widget{a.codeArea}, addons...), width, height)
 		a.TTY.UpdateBuffer(bufNotes, bufMain, flag&fullRedraw != 0)
 	}
 }
@@ -291,18 +292,95 @@ func renderNotes(notes []string, width int) *term.Buffer {
 }
 
 // Renders the codearea, and uses the rest of the height for the listing.
-func renderApp(codeArea tk.Renderer, addons []tk.Widget, width, height int) *term.Buffer {
-	buf := codeArea.Render(width, height)
-	for _, w := range addons {
-		if len(buf.Lines) >= height {
-			break
+func renderApp(widgets []tk.Widget, width, height int) *term.Buffer {
+	heights, focus := distributeHeight(widgets, width, height)
+	var buf *term.Buffer
+	for i, w := range widgets {
+		if heights[i] == 0 {
+			continue
+		}
+		buf2 := w.Render(width, heights[i])
+		if buf == nil {
+			buf = buf2
+		} else {
+			buf.Extend(buf2, i == focus)
 		}
-		bufListing := w.Render(width, height-len(buf.Lines))
-		buf.Extend(bufListing, hasFocus(w))
 	}
 	return buf
 }
 
+// Distributes the height among all the widgets. Returns the height for each
+// widget, and the index of the widget currently focused.
+func distributeHeight(widgets []tk.Widget, width, height int) ([]int, int) {
+	var focus int
+	for i, w := range widgets {
+		if hasFocus(w) {
+			focus = i
+		}
+	}
+	n := len(widgets)
+	heights := make([]int, n)
+	if height <= n {
+		// Not enough (or just enough) height to render every widget with a
+		// height of 1.
+		remain := height
+		// Start from the focused widget, and extend downwards as much as
+		// possible.
+		for i := focus; i < n && remain > 0; i++ {
+			heights[i] = 1
+			remain--
+		}
+		// If there is still space remaining, start from the focused widget
+		// again, and extend upwards as much as possible.
+		for i := focus - 1; i >= 0 && remain > 0; i-- {
+			heights[i] = 1
+			remain--
+		}
+		return heights, focus
+	}
+
+	maxHeights := make([]int, n)
+	for i, w := range widgets {
+		maxHeights[i] = w.MaxHeight(width, height)
+	}
+
+	// The algorithm below achieves the following goals:
+	//
+	// 1. If maxHeights[u] > maxHeights[v], heights[u] >= heights[v];
+	//
+	// 2. While achieving goal 1, have as many widgets u s.t. heights[u] ==
+	//    maxHeights[u].
+	//
+	// This is done by allocating the height among the widgets following an
+	// non-decreasing order of maxHeights. At each step:
+	//
+	// - If it's possible to allocate maxHeights[u] to all remaining widgets,
+	//   then allocate maxHeights[u] to widget u;
+	//
+	// - If not, allocate the remaining budget evenly - rounding down at each
+	//   step, so the widgets with smaller maxHeights gets smaller heights.
+
+	indices := make([]int, n)
+	for i := range indices {
+		indices[i] = i
+	}
+	sort.Slice(indices, func(i, j int) bool {
+		return maxHeights[indices[i]] < maxHeights[indices[j]]
+	})
+
+	remain := height
+	for rank, idx := range indices {
+		if remain >= maxHeights[idx] {
+			heights[idx] = maxHeights[idx]
+		} else {
+			heights[idx] = remain / (n - rank)
+		}
+		remain -= heights[idx]
+	}
+
+	return heights, focus
+}
+
 func hasFocus(w interface{}) bool {
 	if f, ok := w.(interface{ Focus() bool }); ok {
 		return f.Focus()

pkg/cli/app_test.go

@@ -416,8 +416,9 @@ func TestReadCode_HidesAddonsWhenNotEnoughSpace(t *testing.T) {
 		})
 	defer f.Stop()
 
-	f.TestTTY(t, "\n",
-		term.DotHere, "addon1> ")
+	f.TestTTY(t,
+		"addon1> \n",
+		term.DotHere, "addon2> ")
 }
 
 type testAddon struct {

pkg/cli/modes/histwalk.go

@@ -37,13 +37,21 @@ type histwalk struct {
 }
 
 func (w *histwalk) Render(width, height int) *term.Buffer {
-	cmd, _ := w.cursor.Get()
-	content := modeLine(fmt.Sprintf(" HISTORY #%d ", cmd.Seq), false)
-	buf := term.NewBufferBuilder(width).WriteStyled(content).Buffer()
+	buf := w.render(width)
 	buf.TrimToLines(0, height)
 	return buf
 }
 
+func (w *histwalk) MaxHeight(width, height int) int {
+	return len(w.render(width).Lines)
+}
+
+func (w *histwalk) render(width int) *term.Buffer {
+	cmd, _ := w.cursor.Get()
+	content := modeLine(fmt.Sprintf(" HISTORY #%d ", cmd.Seq), false)
+	return term.NewBufferBuilder(width).WriteStyled(content).Buffer()
+}
+
 func (w *histwalk) Handle(event term.Event) bool {
 	handled := w.Bindings.Handle(w, event)
 	if handled {

pkg/cli/modes/instant.go

@@ -32,18 +32,26 @@ type instant struct {
 }
 
 func (w *instant) Render(width, height int) *term.Buffer {
+	buf := w.render(width, height)
+	buf.TrimToLines(0, height)
+	return buf
+}
+
+func (w *instant) MaxHeight(width, height int) int {
+	return len(w.render(width, height).Lines)
+}
+
+func (w *instant) render(width, height int) *term.Buffer {
 	bb := term.NewBufferBuilder(width).
 		WriteStyled(modeLine(" INSTANT ", false)).SetDotHere()
 	if w.lastErr != nil {
 		bb.Newline().Write(w.lastErr.Error(), ui.FgRed)
 	}
 	buf := bb.Buffer()
-	if len(buf.Lines) >= height {
-		buf.TrimToLines(0, height)
-		return buf
+	if len(buf.Lines) < height {
+		bufTextView := w.textView.Render(width, height-len(buf.Lines))
+		buf.Extend(bufTextView, false)
 	}
-	bufTextView := w.textView.Render(width, height-len(buf.Lines))
-	buf.Extend(bufTextView, false)
 	return buf
 }
 

pkg/cli/modes/navigation.go

@@ -99,6 +99,10 @@ func (w *navigation) Render(width, height int) *term.Buffer {
 	return buf
 }
 
+func (w *navigation) MaxHeight(width, height int) int {
+	return w.codeArea.MaxHeight(width, height) + w.colView.MaxHeight(width, height)
+}
+
 func (w *navigation) Focus() bool {
 	return w.CopyState().Filtering
 }

pkg/cli/modes/navigation_test.go

@@ -192,10 +192,8 @@ func testNavigation(t *testing.T, c NavigationCursor) {
 		"     ++++++++++++++                    -",
 		" d    d2                               │\n", Styles,
 		"#### //////////////                    -",
-		" f    d3                                \n", Styles,
+		" f    d3                                ", Styles,
 		"     //////////////                    X",
-		"                                        ", Styles,
-		"                                       X",
 	)
 	f.TTY.TestBuffer(t, d1Buf2)
 

pkg/cli/modes/stub.go

@@ -24,12 +24,20 @@ type stub struct {
 }
 
 func (w stub) Render(width, height int) *term.Buffer {
-	buf := term.NewBufferBuilder(width).
-		WriteStyled(modeLine(w.Name, false)).SetDotHere().Buffer()
+	buf := w.render(width)
 	buf.TrimToLines(0, height)
 	return buf
 }
 
+func (w stub) MaxHeight(width, height int) int {
+	return len(w.render(width).Lines)
+}
+
+func (w stub) render(width int) *term.Buffer {
+	return term.NewBufferBuilder(width).
+		WriteStyled(modeLine(w.Name, false)).SetDotHere().Buffer()
+}
+
 func (w stub) Handle(event term.Event) bool {
 	return w.Bindings.Handle(w, event)
 }

pkg/cli/tk/codearea.go

@@ -146,12 +146,20 @@ func (w *codeArea) Submit() {
 // Render renders the code area, including the prompt and rprompt, highlighted
 // code, the cursor, and compilation errors in the code content.
 func (w *codeArea) Render(width, height int) *term.Buffer {
+	b := w.render(width)
+	truncateToHeight(b, height)
+	return b
+}
+
+func (w *codeArea) MaxHeight(width, height int) int {
+	return len(w.render(width).Lines)
+}
+
+func (w *codeArea) render(width int) *term.Buffer {
 	view := getView(w)
 	bb := term.NewBufferBuilder(width)
 	renderView(view, bb)
-	b := bb.Buffer()
-	truncateToHeight(b, height)
-	return b
+	return bb.Buffer()
 }
 
 // Handle handles KeyEvent's of non-function keys, as well as PasteSetting

pkg/cli/tk/colview.go

@@ -95,26 +95,47 @@ const colViewColGap = 1
 // Render renders all the columns side by side, putting the dot in the focused
 // column.
 func (w *colView) Render(width, height int) *term.Buffer {
+	cols, widths := w.prepareRender(width)
+	if len(cols) == 0 {
+		return &term.Buffer{Width: width}
+	}
+	var buf term.Buffer
+	for i, col := range cols {
+		if i > 0 {
+			buf.Width += colViewColGap
+		}
+		bufCol := col.Render(widths[i], height)
+		buf.ExtendRight(bufCol)
+	}
+	return &buf
+}
+
+func (w *colView) MaxHeight(width, height int) int {
+	cols, widths := w.prepareRender(width)
+	max := 0
+	for i, col := range cols {
+		colMax := col.MaxHeight(widths[i], height)
+		if max < colMax {
+			max = colMax
+		}
+	}
+	return max
+}
+
+// Returns widgets in and widths of columns.
+func (w *colView) prepareRender(width int) ([]Widget, []int) {
 	state := w.CopyState()
 	ncols := len(state.Columns)
 	if ncols == 0 {
 		// No column.
-		return &term.Buffer{Width: width}
+		return nil, nil
 	}
 	if width < ncols {
 		// To narrow; give up by rendering nothing.
-		return &term.Buffer{Width: width}
+		return nil, nil
 	}
-	colWidths := distribute(width-(ncols-1)*colViewColGap, w.Weights(ncols))
-	var buf term.Buffer
-	for i, col := range state.Columns {
-		if i > 0 {
-			buf.Width += colViewColGap
-		}
-		bufCol := col.Render(colWidths[i], height)
-		buf.ExtendRight(bufCol)
-	}
-	return &buf
+	widths := distribute(width-(ncols-1)*colViewColGap, w.Weights(ncols))
+	return state.Columns, widths
 }
 
 // Handle handles the event first by consulting the overlay handler, and then

pkg/cli/tk/combobox.go

@@ -53,6 +53,10 @@ func (w *comboBox) Render(width, height int) *term.Buffer {
 	return buf
 }
 
+func (w *comboBox) MaxHeight(width, height int) int {
+	return w.codeArea.MaxHeight(width, height) + w.listBox.MaxHeight(width, height)
+}
+
 // Handle first lets the listbox handle the event, and if it is unhandled, lets
 // the codearea handle it. If the codearea has handled the event and the code
 // content has changed, it calls OnFilter with the new content.

pkg/cli/tk/empty.go

@@ -12,6 +12,11 @@ func (Empty) Render(width, height int) *term.Buffer {
 	return term.NewBufferBuilder(width).Buffer()
 }
 
+// MaxHeight returns 1, since this widget always occupies one line.
+func (Empty) MaxHeight(width, height int) int {
+	return 1
+}
+
 // Handle always returns false.
 func (Empty) Handle(event term.Event) bool {
 	return false

pkg/cli/tk/label.go

@@ -13,13 +13,21 @@ type Label struct {
 // Render shows the content. If the given box is too small, the text is cropped.
 func (l Label) Render(width, height int) *term.Buffer {
 	// TODO: Optimize by stopping as soon as $height rows are written.
-	bb := term.NewBufferBuilder(width)
-	bb.WriteStyled(l.Content)
-	b := bb.Buffer()
+	b := l.render(width)
 	b.TrimToLines(0, height)
 	return b
 }
 
+// MaxHeight returns the maximum height the Label can take when rendering within
+// a bound box.
+func (l Label) MaxHeight(width, height int) int {
+	return len(l.render(width).Lines)
+}
+
+func (l Label) render(width int) *term.Buffer {
+	return term.NewBufferBuilder(width).WriteStyled(l.Content).Buffer()
+}
+
 // Handle always returns false.
 func (l Label) Handle(event term.Event) bool {
 	return false

pkg/cli/tk/listbox.go

@@ -85,6 +85,25 @@ func (w *listBox) Render(width, height int) *term.Buffer {
 	return w.renderVertical(width, height)
 }
 
+func (w *listBox) MaxHeight(width, height int) int {
+	s := w.CopyState()
+	if s.Items == nil || s.Items.Len() == 0 {
+		return 0
+	}
+	if w.Horizontal {
+		_, h := getHorizontalWindow(s, w.Padding, width, height)
+		return h
+	}
+	h := 0
+	for i := 0; i < s.Items.Len(); i++ {
+		h += s.Items.Show(i).CountLines()
+		if h >= height {
+			return height
+		}
+	}
+	return h
+}
+
 const listBoxColGap = 2
 
 func (w *listBox) renderHorizontal(width, height int) *term.Buffer {

pkg/cli/tk/listbox_window.go

@@ -95,8 +95,8 @@ func getVerticalWindow(state ListBoxState, height int) (first, crop int) {
 	return 0, 0
 }
 
-// Determines the window to show in horizontal  It returns the first item
-// to show and the amount of height required.
+// Determines the window to show in horizontal. Returns the first item to show
+// and the amount of height required.
 func getHorizontalWindow(state ListBoxState, padding, width, height int) (int, int) {
 	items := state.Items
 	n := items.Len()

pkg/cli/tk/textview.go

@@ -80,6 +80,10 @@ func (w *textView) Render(width, height int) *term.Buffer {
 	return buf
 }
 
+func (w *textView) MaxHeight(width, height int) int {
+	return len(w.CopyState().Lines)
+}
+
 func (w *textView) getStateForRender(height int) (lines []string, first int) {
 	w.MutateState(func(s *TextViewState) {
 		if s.First > len(s.Lines)-height && len(s.Lines)-height >= 0 {

pkg/cli/tk/widget.go

@@ -12,15 +12,24 @@ import (
 // render itself.
 type Widget interface {
 	Renderer
+	MaxHeighter
 	Handler
 }
 
 // Renderer wraps the Render method.
 type Renderer interface {
-	// Render onto a region of bound width and height.
+	// Render renders onto a region of bound width and height.
 	Render(width, height int) *term.Buffer
 }
 
+// MaxHeighter wraps the MaxHeight method.
+type MaxHeighter interface {
+	// MaxHeight returns the maximum height needed when rendering onto a region
+	// of bound width and height. The returned value may be larger than the
+	// height argument.
+	MaxHeight(width, height int) int
+}
+
 // Handler wraps the Handle method.
 type Handler interface {
 	// Try to handle a terminal event and returns whether the event has been