bounds when panning by keyboard

This makes the keyboard navigation feel more solid.

I'm not bothering to do the same for panning with the mouse. Mostly
because I'm lazy, but also as an escape hatch in case I find myself
wanting to move the top lower down the screen or something. But it'll
still snap back the first time I pan using the keyboard. Let's see how
much it matters.

0012-on.initialize

@@ -5,4 +5,6 @@ on.initialize = function()
 	font:setLineHeight(1.3)
 	initialize_file_picker()
 	A()
+	Viewport.x, Viewport.y = Viewport_bounds.xmin, Viewport_bounds.ymin
+	B()
 end

0023-on.keychord_press

@@ -32,34 +32,46 @@ on.keychord_press = function(chord, key)
 		end
 	else
 		if chord == 'up' then
-			Viewport.y = Viewport.y - scale(20)
+			Viewport.y = math.max(
+				Viewport_bounds.ymin,
+				Viewport.y - scale(20))
 			B()
 		elseif chord == 'down' then
-			Viewport.y = Viewport.y + scale(20)
+			Viewport.y = math.min(
+				math.max(
+					Viewport_bounds.ymin,
+					Viewport_bounds.ymax - App.screen.width/2/Viewport.zoom),  -- conservative; unclear why removing the '/2' makes the bottom inaccessible
+				Viewport.y + scale(20))
 			B()
 		elseif chord == 'left' then
-			Viewport.x = Viewport.x - scale(50)
+			Viewport.x = math.max(
+				Viewport_bounds.xmin,
+				Viewport.x - scale(50))
 			B()
 		elseif chord == 'right' then
-			Viewport.x = Viewport.x + scale(50)
+			Viewport.x = math.min(
+				math.max(Viewport_bounds.xmin, Viewport_bounds.xmax - App.screen.width/Viewport.zoom),
+				Viewport.x + scale(50))
 			B()
-		elseif chord == 'pageup' then
-			Viewport.y = Viewport.y - App.screen.height/Viewport.zoom
+		elseif chord == 'pageup' or chord == 'S-up' then
+			Viewport.y = math.max(Viewport_bounds.ymin, Viewport.y - App.screen.height/Viewport.zoom)
 			B()
-		elseif chord == 'S-up' then
-			Viewport.y = Viewport.y - App.screen.height/Viewport.zoom
-			B()
-		elseif chord == 'pagedown' then
-			Viewport.y = Viewport.y + App.screen.height/Viewport.zoom
-			B()
-		elseif chord == 'S-down' then
-			Viewport.y = Viewport.y + App.screen.height/Viewport.zoom
+		elseif chord == 'pagedown' or chord == 'S-down' then
+			Viewport.y = math.min(
+				math.max(
+					Viewport_bounds.ymin,
+					Viewport_bounds.ymax - App.screen.width/2/Viewport.zoom),  -- conservative; unclear why removing the '/2' makes the bottom inaccessible
+				Viewport.y + App.screen.height/Viewport.zoom)
 			B()
 		elseif chord == 'S-left' then
-			Viewport.x = Viewport.x - App.screen.width/Viewport.zoom
+			Viewport.x = math.max(Viewport_bounds.xmin, Viewport.x - App.screen.width/Viewport.zoom)
 			B()
 		elseif chord == 'S-right' then
-			Viewport.x = Viewport.x + App.screen.width/Viewport.zoom
+			Viewport.x = math.min(
+				math.max(
+					Viewport_bounds.xmin,
+					Viewport_bounds.xmax - App.screen.width/Viewport.zoom),
+				Viewport.x + App.screen.width/Viewport.zoom)
 			B()
 		end
 	end

0028-A

@@ -8,7 +8,7 @@ A = function(preserve_screen_top_of_cursor_node)
 	else
 		compute_layout(Global_state.thread, 0,0, Surface, preserve_screen_top_of_cursor_node)
 	end
-	-- continue the pipeline
+	compute_viewport_bounds()
 	B(preserve_screen_top_of_cursor_node)
 	-- TODO: ugly that we're manipulating editor objects twice
 end

0117-open_thread

@@ -3,4 +3,6 @@ open_thread = function(filename)
 	load_subtree(filename, Global_state.thread.data, 0)
 	love.window.setTitle('pothi.love - '..filename)
 	A()
+	Viewport.x, Viewport.y = Viewport_bounds.xmin, Viewport_bounds.ymin
+	B()
 end

0147-Viewport_bounds

@@ -0,0 +1 @@
+Viewport_bounds = {}

0148-compute_viewport_bounds

@@ -0,0 +1,21 @@
+compute_viewport_bounds = function()
+	local xmin, ymin, xmax, ymax
+	for i,node in ipairs(Surface) do
+		if node.type == 'text' or node.type == 'rows' or node.type == 'cols' then
+			if xmin == nil or xmin > node.x then
+				xmin = node.x
+			end
+			if xmax == nil or xmax < node.x+node.w then
+				xmax = node.x+node.w
+			end
+			if ymin == nil or ymin > node.y then
+				ymin = node.y
+			end
+			if ymax == nil or ymax < node.y + node.h then
+				ymax = node.y+node.w
+			end
+		end
+	end
+	xmin, ymin = xmin-50, ymin-50
+	Viewport_bounds.xmin, Viewport_bounds.xmax, Viewport_bounds.ymin, Viewport_bounds.ymax = xmin,xmax, ymin, ymax
+end