fix(l2): no bright bar on the dark screen at power-on

The first frame (the sprite's default white background, black dots) came out as
a bright bar across the middle of the screen. A plain black frame now, and the
backlight stays off until it's sent: dark through the panel's reset and init
(the LP5814 keeps its state across a restart), lit with the first frame.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

examples/companion_radio/main.cpp

@@ -224,6 +224,13 @@ void setup() {
   DisplayDriver* disp = NULL;
   if (display.begin()) {
     disp = &display;
+  #if defined(SEEED_WIO_TRACKER_L2)
+    // L2: a plain black frame -- the panel's backlight comes on with it (WioTrackerL2Display
+    // ::endFrame()). The default colours here (white background, black dots) came out as a
+    // bright bar across the middle of the dark screen. LVGL's splash follows within a second.
+    disp->startFrame(0x0000);
+    disp->endFrame();
+  #else
     disp->startFrame();
   #ifdef ST7789
     disp->setTextSize(2);
@@ -234,6 +241,7 @@ void setup() {
     for (int i = -1; i <= 1; i++)
       disp->fillRect(disp->width() / 2 + i * 3 * dot - dot / 2, disp->height() / 2 - dot / 2, dot, dot);
     disp->endFrame();
+  #endif
   }
 #endif
 

variants/wio-tracker-l2/WioTrackerL2Board.cpp

@@ -59,6 +59,15 @@ bool WioTrackerL2Board::initExpander() {
     return false;  // expander not responding
   }
 
+  // The backlight (the LP5814 at 0x2C) dark before the panel is powered and
+  // reset below: until it's initialised the panel shows what's in its memory (a
+  // white bar over the ~0.5 s settle), and the LP5814 keeps its state across a
+  // restart. Standby: CHIP_EN off; the display driver brings it up again.
+  Wire.beginTransmission(0x2C);
+  Wire.write((uint8_t)0x00);
+  Wire.write((uint8_t)0x00);
+  Wire.endTransmission();
+
   expSetInput(EXP_PIN_WAKE_BTN);
   expSetInput(EXP_PIN_I2C_IRQ);
   expSetInput(EXP_PIN_SD_DETECT);

variants/wio-tracker-l2/WioTrackerL2Display.h

@@ -113,8 +113,11 @@ class LGFX_WioTrackerL2 : public lgfx::LGFX_Device {
 public:
   Panel_WioTrackerL2& panelL2() { return _panel_instance; }
   bool init_impl(bool use_reset, bool use_clear) override {
-    // bring up backlight controller while the I2C bus is still clean
-    _light_instance.init(_light_instance.getBrightness());
+    // Bring up the backlight controller while the I2C bus is still clean -- but
+    // dark: the panel's memory holds noise until it's initialised and cleared
+    // (a white bar across the screen), and a chip left lit by a restart would
+    // show it too. begin() switches the light on after the clear.
+    _light_instance.init(0);
 
     bool result = LGFX_Device::init_impl(use_reset, use_clear);
 
@@ -202,6 +205,8 @@ class WioTrackerL2Display : public LGFXDisplay {
     }
   }
 
+  bool _lit = false;   // the light has come on with the first frame
+
 public:
   WioTrackerL2Display() : LGFXDisplay(320, 240, disp) {}
 
@@ -231,6 +236,14 @@ public:
   }
   void fadeIn() { fade(0, display->getBrightness()); }
 
+  // The first frame goes out with the light still off, then the light comes on:
+  // lit any sooner, the panel shows what it holds in memory for as long as the
+  // frame takes.
+  void endFrame() override {
+    LGFXDisplay::endFrame();
+    if (!_lit) { _lit = true; display->setBrightness(153); }
+  }
+
   // direct access to the LGFX device (LVGL flush/touch glue)
   lgfx::LGFX_Device* lgfxDevice() { return &disp; }
   Panel_WioTrackerL2& panel() { return disp.panelL2(); }
@@ -260,8 +273,7 @@ public:
     // pixels keep whatever the panel powered up with (they showed as coloured
     // lines down the right side and along the bottom). Clear the panel once.
     display->fillScreen(TFT_BLACK);
-    display->setBrightness(153);
-    display->setTextColor(TFT_WHITE);
+    display->setTextColor(TFT_WHITE);   // the light comes on with the first frame (endFrame())
 
     buffer.setColorDepth(8);
     buffer.setPsram(true);