click on file picker to open thread

There's one remaining issue: the DOM is changing between mouse press and
mouse release, which is confusing the select handler.

Options:
  * some sort of hacky thing to ignore the first mouse release
  * put the file picker handlers on mouse release rather than mouse
    press

There's some precedent for the latter option even if it might get
confusing in the presence of drag operations. But that seems confusing
either way. Just be consistent.

0006-on.mouse_press

@@ -3,9 +3,22 @@ on.mouse_press = function(x,y, mouse_button)
 		Cursor_node.show_cursor = nil
 		Cursor_node = nil
 	end
+	-- buttons at the HUD level can use the buttons.lua abstraction
 	if mouse_press_consumed_by_any_button_handler(HUD, x,y, mouse_button) then
 		return
 	end
+	-- other buttons need manual handling
+	if Global_state.thread == nil then
+		-- we're rendering the file picker
+		-- compute_layout reuses nodes and so it fills in x,y for us
+		for _,button in ipairs(Global_state.file_picker) do
+			if in_rect(button, x,y) then
+				open_thread(button.data[1].data)
+				A()
+				break
+			end
+		end
+	end
 	local node = on_text(x,y)
 	if node then
 		-- position cursor in node

0023-on.keychord_press

@@ -3,6 +3,7 @@ on.keychord_press = function(chord, key)
 	if Global_state.thread and chord == 'C-o' then
 		Global_state.thread = nil
 		reset_viewport()
+		A()
 	elseif chord == 'C-=' then
 		-- zoom in
 		Viewport.zoom = Viewport.zoom+0.1

0028-A

@@ -3,9 +3,9 @@ A = function(preserve_screen_top_of_cursor_node)
 	love.graphics.setFont(love.graphics.newFont(scale(20)))  -- editor objects implicitly depend on current font
 	-- translate some page in Global_state to Surface
 	Surface = {}
-	if Global_state.file_picker then
+	if Global_state.thread == nil then
 		lay_out_file_picker()
-	elseif Global_state.thread then
+	else
 		compute_layout(Global_state.thread, 0,0, Surface, preserve_screen_top_of_cursor_node)
 	end
 	-- continue the pipeline

0132-in_rect

@@ -0,0 +1,8 @@
+in_rect = function(rect, x,y)
+	-- return true if x,y in viewport coordinates lie with rect on the surface
+	if x < vx(rect.x) then return end
+	if y < vy(rect.y) then return end
+	if x > vx(rect.x+rect.w) then return end
+	if y > vy(rect.y+rect.h) then return end
+	return true
+end