feat(core): restart loader from current position

On "hold to confirm" dialogs, when you hold your finger, lift it, and
quickly hold again, the progress bar jumps to the beginning. This commit
changes the behavior so the progress continues from its current position
of the reverse animation.

core/src/apps/homescreen/homescreen.py

@@ -32,6 +32,7 @@ class Homescreen(HomescreenBase):
             style=LoaderNeutral,
             target_ms=_LOADER_TOTAL_MS - _LOADER_DELAY_MS,
             offset_y=-10,
+            reverse_speedup=3,
         )
         self.touch_ms: Optional[int] = None
 

core/src/trezor/ui/loader.py

@@ -44,6 +44,7 @@ if False:
 
 _TARGET_MS = const(1000)
 _OFFSET_Y = const(-24)
+_REVERSE_SPEEDUP = const(2)
 
 
 class Loader(ui.Component):
@@ -52,6 +53,7 @@ class Loader(ui.Component):
         style: LoaderStyleType = LoaderDefault,
         target_ms: int = _TARGET_MS,
         offset_y: int = _OFFSET_Y,
+        reverse_speedup: int = _REVERSE_SPEEDUP,
     ) -> None:
         super().__init__()
         self.normal_style = style.normal
@@ -60,9 +62,13 @@ class Loader(ui.Component):
         self.start_ms: Optional[int] = None
         self.stop_ms: Optional[int] = None
         self.offset_y = offset_y
+        self.reverse_speedup = reverse_speedup
 
     def start(self) -> None:
-        self.start_ms = utime.ticks_ms()
+        if self.start_ms is not None and self.stop_ms is not None:
+            self.start_ms = utime.ticks_ms() - self.elapsed_ms()
+        else:
+            self.start_ms = utime.ticks_ms()
         self.stop_ms = None
         self.on_start()
 
@@ -70,27 +76,27 @@ class Loader(ui.Component):
         self.stop_ms = utime.ticks_ms()
 
     def elapsed_ms(self) -> int:
-        if self.start_ms is None:
-            return 0
-        return utime.ticks_ms() - self.start_ms
-
-    def on_render(self) -> None:
-        target = self.target_ms
         start = self.start_ms
         stop = self.stop_ms
-        if start is None:
-            return
         now = utime.ticks_ms()
-        if stop is None:
-            r = min(now - start, target)
+        if start is None:
+            return 0
+        elif stop is not None:
+            return max(stop - start + (stop - now) * self.reverse_speedup, 0)
         else:
-            r = max(stop - start + (stop - now) * 2, 0)
+            return min(now - start, self.target_ms)
+
+    def on_render(self) -> None:
+        if self.start_ms is None:
+            return
+        target = self.target_ms
+        r = self.elapsed_ms()
         if r != target:
             s = self.normal_style
         else:
             s = self.active_style
 
-        progress = r * 1000 // target
+        progress = r * 1000 // target  # scale to 0-1000
         if s.icon is None:
             display.loader(progress, False, self.offset_y, s.fg_color, s.bg_color)
         else: