Media: the player's bar fades by elapsed time, so its scrubber and buttons keep their state

The fade restarted an animation id at each rest and wake, and gpui hands
that id to every descendant: the scrubber's drag, hover and focus handle
and the buttons' focus were rebuilt under it. The bar's opacity now reads
the time since the last change through motion::since_change. The stage
tracks focus even without key handlers, so a focused bar button keeps
the bar up. The player's tests move to their own file.

src/media/player.rs

@@ -1,9 +1,8 @@
 use std::{rc::Rc, time::Duration};
 
 use gpui::{
-    Animation, AnimationExt, App, ElementId, ImageSource, InteractiveElement, IntoElement,
-    ParentElement, RenderOnce, SharedString, Styled, Window, div, img, prelude::*,
-    transparent_black,
+    App, ElementId, ImageSource, InteractiveElement, IntoElement, ParentElement, RenderOnce,
+    SharedString, Styled, Window, div, img, prelude::*, transparent_black,
 };
 
 use super::scrubber::{OnTime, Scrubber, clock};
@@ -155,7 +154,13 @@ impl RenderOnce for VideoPlayer {
         let resting = self.playing
             && now.duration_since(watch.read(cx).moved) >= IDLE
             && !focus.contains_focused(window, cx);
-        let changes = crate::motion::changes((self.id.clone(), "shown"), !resting, window, cx);
+        let fading = crate::motion::since_change(
+            &(self.id.clone(), "fade").into(),
+            resting,
+            motion_duration(crate::motion::BASE, cx),
+            window,
+            cx,
+        );
         let theme = cx.theme();
         let colors = theme.colors.clone();
         let light = colors.on_media;
@@ -294,18 +299,8 @@ impl RenderOnce for VideoPlayer {
         let bar = bar.when(resting, |bar| {
             bar.debug_selector(|| "video-controls-resting".into())
         });
-        let (from, to) = if resting { (1.0, 0.0) } else { (0.0, 1.0) };
-        let bar = if changes == 0 {
-            bar.opacity(to).into_any_element()
-        } else {
-            let fade = motion_duration(crate::motion::BASE, cx);
-            bar.with_animation(
-                (self.id.clone(), format!("controls-{changes}")),
-                Animation::new(fade),
-                move |bar, t| bar.opacity(crate::motion::lerp(from, to, t)),
-            )
-            .into_any_element()
-        };
+        let shown = if resting { 0.0 } else { 1.0 };
+        let bar = bar.opacity(fading.map_or(shown, |t| crate::motion::lerp(1.0 - shown, shown, t)));
         let controls = div()
             .absolute()
             .left_0()
@@ -333,7 +328,8 @@ impl RenderOnce for VideoPlayer {
             .bg(colors.media_backdrop)
             .border_1()
             .border_color(transparent_black())
-            .when(keys, |stage| stage.track_focus(&focus).focus_ring(cx))
+            .track_focus(&focus)
+            .when(keys, |stage| stage.focus_ring(cx))
             .children(frame)
             .child(controls)
             .on_mouse_move(move |_, window, cx| {