ensure cursor is initially on viewport

Important with larger threads like the input2 example.

0652-fwmanifest

@@ -0,0 +1 @@
+{"render_thread_to_surface":652,"dehtml":456,"Cursor_node":172,"on.code_change":578,"copy_shape":396,"on.text_input":587,"header":651,"Surface":588,"update_editor_box":430,"load_from_iterator":463,"on.draw":632,"compute_ntracks":598,"on":1,"Viewport":303,"font":353,"split_lines":469,"schema1_of_y":366,"on.mouse_press":618,"y_of_schema1":364,"on.initialize":446,"on.update":368,"scale":7,"add_node":650,"render_node_and_descendants":646,"vx":5,"add_edge":575,"initialize_editor":338,"vy":8,"Nodes":593,"add_thick_line":400,"ntracks":600,"Input_filename":436,"compute_layout":619,"to_text":180,"A":582,"on.keychord_press":644,"box_height":345,"line_height":365,"B":379,"fw_parent":651,"to_node":611,"fw_app":"mastodon-unfurl","ensure_cursor_node_within_viewport":643,"on.mouse_release":586}

0652-render_thread_to_surface

@@ -0,0 +1,50 @@
+render_thread_to_surface = function(thread_data)
+	-- design constraints:
+	--   trees only, no graphs or DAGs
+	--   parents above children
+	--   a child shares its parent's column exactly only if it's an only child
+	--   parents can overlap columns with children and more distant descendants
+	--   siblings never share a column
+	--   siblings never overlap columns
+	--   siblings always occupy the same row
+	--   cousins/aunts/nephews never overlap columns
+	Surface = {}
+	-- we're going to be computing box heights
+	love.graphics.setFont(love.graphics.newFont(scale(20)))
+	-- compute mapping from ids to nodes
+	Nodes = {}  -- id to object
+	Nodes[thread_data.id] = thread_data
+	for _,x in ipairs(thread_data.descendants) do
+		Nodes[x.id] = x
+	end
+	local root = thread_data
+	Cursor_node = root
+	-- compute children
+	for _,x in pairs(Nodes) do
+		parent_id = x.in_reply_to_id
+		if x.id ~= root.id then
+			assert(parent_id)
+			local parent = Nodes[parent_id]
+			if parent.children == nil then
+				parent.children = {}
+			end
+			table.insert(parent.children, x.id)
+		end
+	end
+	-- sort children by time
+	for _,x in pairs(Nodes) do
+		if x.children then
+			table.sort(x.children)
+		end
+	end
+	-- compute number of tracks needed
+	for _,x in pairs(Nodes) do
+		if x.ntracks == nil then
+			x.ntracks = compute_ntracks(x)
+		end
+	end
+	-- prepare the tracks
+	-- each track is 600px + 20px of gutter between nodes
+	render_node_and_descendants(root.id, --[[y]] 0, --[[xlo]] 0, --[[xhi]] 620 * root.ntracks)
+	ensure_cursor_node_within_viewport()
+end

head

@@ -1 +1 @@
-651
+652