fix(settings): paint the scan and keyboard notices before the work that blocks

on_tick runs before draw in the same pass, so a screen built for a blocking call was
never painted until the call returned -- on a panel ui.screen() had already cleared.
Scanning looked like a hang on a blank screen. The notice is now painted by the tap
itself and centered on both axes.

Makefile

@@ -5,7 +5,7 @@ LUA ?= lua
 CXX ?= c++
 CXXFLAGS ?= -std=c++11 -Wall -Wextra
 BUILD_DIR := .pio/build/esp32-32e
-LUA_TESTS := test/ui_layout.lua test/ui_theme.lua test/settings_calibration.lua test/statusbar_dirty.lua
+LUA_TESTS := test/ui_layout.lua test/ui_theme.lua test/settings_calibration.lua test/statusbar_dirty.lua test/settings_busy.lua
 
 .PHONY: test test-lua test-cpp test-stubs stubs build upload monitor clean
 

sdcard/apps/settings/main.lua

@@ -155,10 +155,18 @@ function buildMenu()
   themed(items)
 end
 
+-- Painted here rather than left to draw(): the work these announce blocks the loop, and
+-- on_tick runs before draw in the same pass, so the panel would sit on the background
+-- ui.screen() cleared it to until the blocking call returned.
+local function busy(text)
+  themed{w = "fill", h = "fill", justify = "center", align = "center", ui.label(text)}
+  screen:draw()
+end
+
 local function requestScan()
   mode = "scanning"
   scanRequested = true
-  themed{pad = 12, gap = 8, ui.text("wifi"), ui.text("scanning...")}
+  busy("scanning networks...")
 end
 
 local function forgetNetwork()
@@ -219,7 +227,7 @@ local function chooseNetwork(network)
   end
   keyboardRequested = true
   mode = "password"
-  themed{pad = 12, ui.text("opening keyboard...")}
+  busy("opening keyboard...")
 end
 
 local function selectNetwork(button)

test/settings_busy.lua

@@ -0,0 +1,49 @@
+-- Run: lua test/settings_busy.lua
+-- The scan and keyboard screens announce themselves before work that blocks the loop for
+-- seconds. on_tick runs before draw in the same pass, so anything left for draw() to paint
+-- appears only after the blocking call returns -- on a panel ui.screen() has already
+-- cleared. These assert the announcement is painted by the tap itself.
+package.path = "sdcard/lib/?.lua;test/?.lua;" .. package.path
+
+local device = require("fake_device").install()
+
+dofile("sdcard/apps/settings/main.lua")
+
+local function tapRow(prefix)
+  device.painted = {}
+  draw()
+  for _, item in ipairs(device.painted) do
+    if item.label:sub(1, #prefix) == prefix then
+      device.painted = {}
+      on_touch_down(item.x + 2, item.y + 2)
+      on_touch_up(item.x + 2, item.y + 2)
+      return
+    end
+  end
+  error("no row labelled '" .. prefix .. "'")
+end
+
+local function paintedLabel(prefix)
+  for _, item in ipairs(device.painted) do
+    if item.label:sub(1, #prefix) == prefix then return item end
+  end
+  return nil
+end
+
+init()
+tapRow("wifi")
+tapRow("scan networks")
+
+-- Painted by the tap, with no draw() in between.
+local busy = paintedLabel("scanning")
+assert(busy, "the scan screen paints its notice before scanning")
+
+local width = #busy.label * device.charWidth
+local centerX = busy.x + width / 2
+local centerY = busy.y + device.fontHeight / 2
+local panelW, panelH = gui.getWidth(), gui.getHeight()
+assert(math.abs(centerX - panelW / 2) <= 1, "centered horizontally, got x " .. busy.x)
+-- The bar is above the app's frame, so the app's own height is what it centers in.
+assert(math.abs(centerY - panelH / 2) <= 1, "centered vertically, got y " .. busy.y)
+
+print("ok")