commit 0ca5bb0e8d did indeed cause the regression

Unscaled y makes cursor scrolling more stable, but it seems to be
compensating for an error elsewhere. The location of the top margin of
text is not stable, and I only notice when I can see the bottom margin
of the bounding box. (When the top margin is visible we never enter that
branch in update_editor_box.)

I should just use LÖVE's standard translation and scaling transforms for
the surface! Don't know why I didn't think of that.

0008-Viewport

@@ -1 +1 @@
-Viewport = {x=-50, y=-50, w=800,h=600, zoom=1.0}
+Viewport = {x=-50, y=-50, w=800,h=600, zoom=0.4}

0011-box_height

@@ -1,16 +1,16 @@
 box_height = function(node)
-	-- return the height of a text editor node (explicit width). The result is unscaled.
-	local y = 0
+	-- return the height of a text editor node (explicit width). The result is scaled.
+	local count = 0
 	for i=1,#node.editor.lines do
 		local line = node.editor.lines[i]
 		if node.editor.line_cache[i] == nil then
 			node.editor.line_cache[i] = {}
 		end
-		node.editor.line_cache[i].fragments = nil
 		node.editor.line_cache[i].screen_line_starting_pos = nil
 		Text.populate_screen_line_starting_pos(node.editor, i)
-		y = y + node.editor.line_height*#node.editor.line_cache[i].screen_line_starting_pos
+		count = count + #node.editor.line_cache[i].screen_line_starting_pos
 		Text.clear_screen_line_cache(node.editor, i)
 	end
-	return y
+	print(count, '*', node.editor.line_height, '=', count*node.editor.line_height)
+	return count*node.editor.line_height/Viewport.zoom
 end

0019-B

@@ -1,4 +1,5 @@
 B = function()
+	print('B')
 	-- recompute various aspects based on the current viewport settings
 	for _,obj in ipairs(Surface) do
 		if obj.type == 'line' then

0020-Page

@@ -9,37 +9,68 @@ Page = {
         doc='prose goes here, on the left half of the window',
         margin=Margin_left,
         data={
-			"Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.",
-			'1',
-			'2',
-			'3',
-			'mno',
-			'Acb',
-			'g',
-			'hij',
-			'klm',
-			'nop',
+'1',
+'2',
+'3',
+'4',
+'5',
+'6',
+'7',
+'8',
+'9',
+'10',
+'11',
+'12',
+'13',
+'14',
+'15',
+'16',
+'17',
+'18',
+'19',
+'20',
+'21',
+'22',
+'23',
+'24',
+'25',
+'26',
+'27',
+'28',
+'29',
+'30',
+'31',
+'32',
+'33',
+'34',
+'35',
+'36',
+'37',
+'38',
+'39',
+'40',
+'41',
+'42',
+'43',
+'44',
+'45',
+'46',
+'47',
+'48',
+'49',
+'50',
+'51',
+'52',
+'53',
+'54',
+'55',
+'56',
+'57',
+'58',
+'59',
+'60',
 		},
         width=400, bg={r=1,g=1,b=0}
       },
-      -- a table on the right
-      { type='rows', name='searches', margin=50, data={
-        { type='text', data={''},},
-        { type='cols', data={
-          { type='text', data={'search:'},},
-          { type='text', name='search', bg={r=0.8,g=0.8,b=0.8}, data={''}, width=90,},
-        }},
-		{ type='text', data={'table:'},},
-        { type='cols', bg={r=0.8,g=0.8,b=0.8}, data={
-            { type='rows', width=90, data={
-              {type='text', data={'abc'},},
-              {type='text', data={'abc'},},
-            }},
-            { type='rows', width=90, data={
-              {type='text', data={'def'},},
-              {type='text', data={'def'},},
-            }},
-        }},
-      }},
     },
-}
+}

0026-on.draw

@@ -1,5 +1,10 @@
 on.draw = function()
-	love.graphics.setColor(1,0,0)
+	love.graphics.setColor(0,0,0)
+	love.graphics.line(vx(0), vy(0), vx(0), vy(1000000))
+	love.graphics.line(vx(0), vy(0), vx(1000000), vy(0))
+	for i=1,100 do
+		love.graphics.print(tostring(i), vx(0), vy(i*1000))
+	end
 	for _,obj in ipairs(Surface) do
 		love.graphics.setColor(obj.r or 0, obj.g or 0, obj.b or 0)
 		if obj.type == 'rectangle' then
@@ -18,6 +23,7 @@ on.draw = function()
 			if obj.w == nil then
 				love.graphics.draw(obj.text, vx(obj.x), vy(obj.y))
 			else
+				print(obj.x, obj.y, obj.editor.top, obj.editor.screen_top1.line)
 				edit.draw(obj.editor, obj.fg or {r=0,g=0,b=0}, not obj.show_cursor)
 			end
 		end

0027-Surface

@@ -1,12 +1,2 @@
 Surface = {
-  -- test data
-  {type='line', data={0,-1000, 0,1000}},
-  {type='line', data={-10000,0, 10000,0}},
-  {type='text', data={'0'}, x=-20,y=-30},
-  {type='rectangle', x=50,y=50, w=20,h=80, r=1,g=0,b=0},
-  {type='text', data={'abc', 'def'}, x=150, y=50, w=50,h=50, fg={r=0,g=0.4, b=0.9}},
-  {type='circle', x=300,y=200, radius=40, r=1,g=0,b=1},
-  {type='arc', x=0,y=0, radius=50, angle1=0, angle2=math.pi*2/3},
-  {type='ellipse', x=100,y=100, radiusx=10, radiusy=50},
-  {type='bezier', data={25,25, 25,125, 75,25, 125,25}},
-}
+}

0028-A

@@ -1,16 +1,8 @@
 A = function()
+	print('A')
 	love.graphics.setFont(love.graphics.newFont(scale(20)))  -- editor objects implicitly depend on current font
 	-- translate Page to Surface
-	while #Surface > 3 do table.remove(Surface) end  -- HACK
-	local red = false
-	for x=-1000,2000,300 do
-		for y=-10000,10000,200 do
-			add_thick_line({type='line', data={x,y, x+200,y+200, x,y+400}, r=red and 1 or 0,g=red and 0 or 0.5,b=0}, 10)
-			red = not red
-		end
-	end
 	compute_layout(Page, Page.x,Page.y, Surface)
-	compute_layout(Page2, Page2.x,Page2.y, Surface)
 	-- continue the pipeline
 	B()
 	-- TODO: ugly that we're manipulating editor objects twice

0029-Page2

@@ -1,9 +0,0 @@
-Page2 = {
-	x=500, y=300,
-    -- page
-		type='text',
-        data={
-"Call me Ishmael.  Some years ago--never mind how long precisely--having little or no money in my purse, and nothing particular to interest me on shore, I thought I would sail about a little and see the watery part of the world.  It is a way I have of driving off the spleen and regulating the circulation.  Whenever I find myself growing grim about the mouth; whenever it is a damp, drizzly November in my soul; whenever I find myself involuntarily pausing before coffin warehouses, and bringing up the rear of every funeral I meet; and especially whenever my hypos get such an upper hand of me, that it requires a strong moral principle to prevent me from deliberately stepping into the street, and methodically knocking people's hats off--then, I account it high time to get to sea as soon as I can.  This is my substitute for pistol and ball.  With a philosophical flourish Cato throws himself upon his sword; I quietly take to the ship.  There is nothing surprising in this.  If they but knew it, almost all men in their degree, some time or other, cherish very nearly the same feelings towards the ocean with me."
-		},
-        width=400, bg={r=0,g=0.8,b=0}
-}

0030-update_editor_box

@@ -7,7 +7,7 @@ update_editor_box = function(node)
 		end
 		node.editor.top = vy(node.y)
 	else
-		node.editor.screen_top1, node.editor.top = schema1_of_y(node.editor, Viewport.y-node.y)
+		node.editor.screen_top1, node.editor.top = schema1_of_y(node.editor, scale(Viewport.y-node.y))
 	end
 	node.editor.left = math.floor(vx(node.x))
 	node.editor.right = math.ceil(vx(node.x+node.w))