snapshot: a cleaner organization

scenarios:
  * Zoom = 1
    * pan with mouse: ✓
    * pan with up arrow: ✓
    * pan with down arrow: ✓
  * Zoom < 1
    * pan with mouse: ✓
    * pan with up arrow: ✓
    * pan with down arrow: ✗
  * Zoom > 1
    * pan with mouse: ✓
    * pan with up arrow: ✗
    * pan with down arrow: ✓

What ✓ means:
* pan with mouse: lines don't slide relative to the surface
  * will still slide relative to the surface when zooming in/out;
    that's unavoidable because we want integer pixels for crisp text
* pan with keyboard: at least some part of cursor is always peeking within the viewport
  * might still look ugly, with the line containing the cursor almost invisible,
    but hitting the down arrow will never pan upwards, or vice versa

Still not working though. I'm pretty much guaranteeing by construction that if
Viewport.y was set from screen_top1, then screen_top1 will not be perturbed.
And yet using scale() inside update_editor_box is incorrect. Hmm..

0019-B

@@ -1,4 +1,4 @@
-B = function(preserve_screen_top_of_cursor_node)
+B = function(skip_updating_screen_top_for)
 	-- recompute various aspects based on the current viewport settings
 	for _,obj in ipairs(Surface) do
 		if obj.type == 'line' then
@@ -16,10 +16,10 @@ B = function(preserve_screen_top_of_cursor_node)
 			obj.zdata = love.math.newBezierCurve(zdata):render()
 		elseif obj.type == 'text' then
 			if obj.w then
-				update_editor_box(obj, preserve_screen_top_of_cursor_node)
+				update_editor_box(obj, skip_updating_screen_top_for)
 			else
 				obj.text = love.graphics.newText(love.graphics.getFont(), obj.data)
 			end
 		end
 	end
-end
+end

0021-compute_layout

@@ -1,4 +1,4 @@
-compute_layout = function(node, x,y, nodes_to_render, preserve_screen_top_of_cursor_node)
+compute_layout = function(node, x,y, nodes_to_render, skip_updating_screen_top_for)
 	-- append to nodes_to_render flattened instructions to render a hierarchy of nodes
 	-- return x,y rendered until (surface coordinates)
 	if node.type == 'text' then
@@ -31,7 +31,7 @@ compute_layout = function(node, x,y, nodes_to_render, preserve_screen_top_of_cur
 		if node.editor == nil then
 			initialize_editor(node)
 		else
-			update_editor_box(node, preserve_screen_top_of_cursor_node)
+			update_editor_box(node, skip_updating_screen_top_for)
 		end
 		node.h = box_height(node)
 		table.insert(nodes_to_render, node)

0022-on.text_input

@@ -4,7 +4,9 @@ on.text_input = function(t)
 		edit.text_input(Cursor_node.editor, t)
 		if not eq(Cursor_node.editor.screen_top1, old_top) then
 			Viewport.y = Cursor_node.y + y_of_schema1(Cursor_node.editor, Cursor_node.editor.screen_top1)
+			A(--[[skip updating screen_top for]] Cursor_node)
+			return
 		end
-		A(--[[preserve screen_top of cursor node]] true)
+		A()
 	end
-end
+end

0023-on.keychord_press

@@ -21,8 +21,10 @@ on.keychord_press = function(chord, key)
 		edit.keychord_press(Cursor_node.editor, chord, key)
 		if not eq(Cursor_node.editor.screen_top1, old_top) then
 			Viewport.y = Cursor_node.y + y_of_schema1(Cursor_node.editor, Cursor_node.editor.screen_top1)
+			A(--[[skip updating screen_top for]] Cursor_node)
+			return
 		end
-		A(--[[preserve screen_top of cursor node]] true)
+		A()
 	else
 		if chord == 'up' then
 			Viewport.y = Viewport.y - scale(20)
@@ -56,4 +58,4 @@ on.keychord_press = function(chord, key)
 			B()
 		end
 	end
-end
+end

0028-A

@@ -1,8 +1,8 @@
-A = function(preserve_screen_top_of_cursor_node)
+A = function(skip_updating_screen_top_for)
 	love.graphics.setFont(love.graphics.newFont(scale(20)))  -- editor objects implicitly depend on current font
 	Surface = {}
-	compute_layout(Page, Page.x,Page.y, Surface)
+	compute_layout(Page, Page.x,Page.y, Surface, skip_updating_screen_top_for)
 	-- continue the pipeline
-	B(preserve_screen_top_of_cursor_node)
+	B(skip_updating_screen_top_for)
 	-- TODO: ugly that we're manipulating editor objects twice
 end

0030-update_editor_box

@@ -1,14 +1,14 @@
-update_editor_box = function(node, preserve_screen_top_of_cursor_node)
+update_editor_box = function(node, skip_updating_screen_top_for)
 	if node.editor == nil then return end
 	edit.update_font_settings(node.editor, scale(20))
-	if node.y > Viewport.y then
-		if node ~= Cursor_node then
+	if node ~= skip_updating_screen_top_for then
+		if node.y > Viewport.y then
 			node.editor.screen_top1.line = 1
 			node.editor.screen_top1.pos = 1
+			node.editor.top = vy(node.y)
+		else
+			node.editor.screen_top1, node.editor.top = schema1_of_y(node.editor, scale(Viewport.y-node.y))
 		end
-		node.editor.top = vy(node.y)
-	else
-		node.editor.screen_top1, node.editor.top = schema1_of_y(node.editor, Viewport.y-node.y)
 	end
 	node.editor.left = math.floor(vx(node.x))
 	node.editor.right = math.ceil(vx(node.x+node.w))