seed: use sync backlight sliding

src/apps/common/seed.py

@@ -60,7 +60,7 @@ async def get_keychain(ctx: wire.Context, namespaces: list) -> Keychain:
     return keychain
 
 
-@ui.layout
+@ui.layout_no_slide
 async def _compute_seed(ctx: wire.Context) -> bytes:
     passphrase = cache.get_passphrase()
     if passphrase is None:
@@ -73,9 +73,11 @@ async def _compute_seed(ctx: wire.Context) -> bytes:
 
 
 def _start_bip39_progress():
+    ui.backlight_slide_sync(ui.BACKLIGHT_DIM)
     ui.display.clear()
     ui.header("Please wait")
     ui.display.refresh()
+    ui.backlight_slide_sync(ui.BACKLIGHT_NORMAL)
 
 
 def _render_bip39_progress(progress: int, total: int):

src/trezor/ui/__init__.py

@@ -110,6 +110,13 @@ async def backlight_slide(val: int, delay: int = 35000, step: int = 20):
         yield sleep
 
 
+def backlight_slide_sync(val: int, delay: int = 35000, step: int = 20):
+    current = display.backlight()
+    for i in range(current, val, -step if current > val else step):
+        display.backlight(i)
+        utime.sleep_us(delay)
+
+
 def layout(f):
     async def inner(*args, **kwargs):
         await backlight_slide(BACKLIGHT_DIM)
@@ -127,6 +134,18 @@ def layout(f):
     return inner
 
 
+def layout_no_slide(f):
+    async def inner(*args, **kwargs):
+        try:
+            layout = f(*args, **kwargs)
+            workflow.onlayoutstart(layout)
+            return await layout
+        finally:
+            workflow.onlayoutclose(layout)
+
+    return inner
+
+
 def header(
     title: str, icon: bytes = ICON_DEFAULT, fg: int = FG, bg: int = BG, ifg: int = GREEN
 ):