Add code components 366-385; ScrollArea Follow, editor sync

A terminal with history, a Go-colored code block, a log viewer that
follows, a hex viewer, a memory map, a JSON tree, a line diff, a GAME
OVER crash screen, an FPS meter, profiler bars, breakpoints,
variables, command history, a shortcut list, a debug grid, a pixel
editor that exports string art, a palette editor, a sprite sheet, a
glyph map and a 6502 register view.

layout.ScrollArea takes Follow, and ChatLog now uses it. data.TreeNode
takes a Value drawn in its Slot. pixel gains Has, and Sprite.Art
writes MustSprite art. pixel.Editor takes the caller's value at each
input event, so a value changed between frames is never typed onto.

layout/follow_test.go

@@ -0,0 +1,52 @@
+package layout_test
+
+import (
+	"testing"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/layout"
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+func TestScrollAreaFollowsItsEndWhileThere(t *testing.T) {
+	s, n, follow := ui.ScrollState{}, 10, true
+	tt := ui.NewTester(func(c *ui.Context) {
+		pixel.Use(c, pixel.Settings{Palette: pixel.SpringDay})
+		layout.ScrollArea(c, &s, layout.ScrollAreaOptions{Label: "Log", Follow: follow}, func() {
+			for range n {
+				ui.Box(c).Height(32)
+			}
+		}).Element.Size(200, 100)
+	}, 300, 200)
+	tt.Frame()
+	if s.MaxY == 0 || s.Y != s.MaxY {
+		t.Fatalf("a following view opened at %v of %v", s.Y, s.MaxY)
+	}
+	n++
+	tt.Frame()
+	if s.Y != s.MaxY {
+		t.Fatalf("at the end, a new row left the view at %v of %v", s.Y, s.MaxY)
+	}
+	// Scrolled up it stays; back at the end, it follows.
+	s.Y = 0
+	n++
+	tt.Frame()
+	if s.Y != 0 {
+		t.Fatalf("scrolled up, the view jumped to %v", s.Y)
+	}
+	s.Y = s.MaxY
+	tt.Frame()
+	n++
+	tt.Frame()
+	if s.Y != s.MaxY {
+		t.Fatalf("back at the end, the view stopped following at %v of %v", s.Y, s.MaxY)
+	}
+	// Without Follow, growth leaves the view alone.
+	follow, s = false, ui.ScrollState{}
+	n++
+	tt.Frame()
+	if s.Y != 0 {
+		t.Fatalf("a plain view followed to %v", s.Y)
+	}
+}

layout/scrollarea.go

@@ -25,7 +25,9 @@ var (
 
 type ScrollAreaOptions struct {
 	// Label names the region for assistive technology.
-	Label    string
+	Label string
+	// Follow keeps the view at the end while there.
+	Follow   bool
 	Disabled bool
 }
 
@@ -37,17 +39,32 @@ type ScrollAreaResult struct {
 
 type scrollKey struct{}
 
+// area is the last view and the end last reached.
+type area struct {
+	last, end float32
+}
+
 // ScrollArea scrolls children vertically beside a pixel scrollbar.
 func ScrollArea(c *ui.Context, s *ui.ScrollState, o ScrollAreaOptions, children func()) ScrollAreaResult {
 	k := pixel.K(c)
 	// The bar comes first: TrackScroll reads s when called.
 	row := ui.Row(c).AlignItems(ui.Stretch).Reverse()
-	last := ui.Local(row, scrollKey{}, func() float32 { return s.Y })
+	st := ui.Local(row, scrollKey{}, func() area { return area{last: s.Y} })
 	view := row.Bounds().H
 	// Off freezes the view: any move is undone before paint.
 	inert := pixel.Current(c).Inert
-	if o.Disabled || inert {
-		s.Y = *last
+	off := o.Disabled || inert
+	if off {
+		s.Y = st.last
+	}
+	// Content grew under a view at its old end: follow.
+	grew := func() bool { return o.Follow && s.MaxY > st.end && s.Y >= st.end }
+	// Off, the end waits until it wakes.
+	if !off {
+		if grew() {
+			s.Y = s.MaxY
+		}
+		st.end = s.MaxY
 	}
 	var v *ui.Element
 	row.Children(func() {
@@ -69,7 +86,8 @@ func ScrollArea(c *ui.Context, s *ui.ScrollState, o ScrollAreaOptions, children
 	})
 	focus := v.FocusVisible()
 	row.Draw(func(p *ui.Painter, r ui.Rect) {
-		if r.H != view {
+		// Layout has just measured; resize or follow next frame.
+		if r.H != view || !off && grew() {
 			c.Invalidate()
 		}
 	}).DrawOver(func(p *ui.Painter, r ui.Rect) {
@@ -82,8 +100,8 @@ func ScrollArea(c *ui.Context, s *ui.ScrollState, o ScrollAreaOptions, children
 			g.Checker(w-1, 0, 1, g.H, pixel.Accent)
 		}
 	})
-	res := ScrollAreaResult{Element: row, Changed: s.Y != *last}
-	*last = s.Y
+	res := ScrollAreaResult{Element: row, Changed: s.Y != st.last}
+	st.last = s.Y
 	if res.Changed {
 		slog.Debug("8bit: scrollarea scrolled", "y", s.Y)
 	}