Port · Parity: every page matches gpui 0.2.2's; the rules re-tested

Every page and scripted state was shot with gpui 0.2.2 and with the
port, light and dark, full width and at 280px, judged pair by pair by
agents with skeptics, and every page's log diffed. What differed is
fixed, each with a test that fails without it where the test platform
can show it:

- The gallery's harness takes posted mouse moves (gpui_macos listens
  to a tracking area), posts each drag move twice (it no longer
  replays a held drag), and waits three frames before each shot.
- Rolling digits: a block column, so taffy 0.13 lends the cell no
  baseline, and a line of whole device pixels, since gpui snaps each
  length on its own. DescriptionList's row aligns ends.
- Margins that taffy 0.13 collapses through a plain block: the
  drawer's pull and snap, the stagger and transition rise, the dialog
  and palette rises, the fullscreen dialog's slide.
- An Input keeps room to type in a squeezed row; the field rows drop
  w_full, whose reason was taffy 0.9's.

AGENTS.md's rules that named gpui 0.2.2 or taffy 0.9 were re-tested
against both sources and rewritten, and new ones say what the port
changed. TASKS.md's tags follow: what the new gpui makes possible is
prove. Captions, doc comments and README say what holds now.

src/typography/fit.rs

@@ -111,7 +111,7 @@ fn offset(align: TextAlign, free: Pixels) -> Pixels {
     }
 }
 
-/// One line in the text style around it, cut by `fit` to its box and set by its alignment. gpui 0.2.2 keeps a line's first measure, so its own `truncate` clips instead in flex boxes.
+/// One line in the text style around it, cut by `fit` to its box and set by its alignment.
 fn cut_line(text: SharedString, fit: fn(&str, Pixels, &mut Window) -> String) -> Div {
     let line = text.clone();
     div()

src/typography/rolling.rs

@@ -124,8 +124,11 @@ impl RenderOnce for AnimatedNumber {
         let duration = motion::duration(motion::SLOW * 2, cx);
         let theme = cx.theme();
         let size = theme.text_size(self.size);
-        let line = size.to_pixels(window.rem_size()) * LEADING;
+        let scale = window.scale_factor();
+        // gpui snaps rows and margin apart; keep whole device pixels.
+        let line = (size.to_pixels(window.rem_size()) * LEADING * scale).round() / scale;
         let row = tabular(div())
+            .debug_selector(|| format!("animated-number {}", self.id))
             .id(self.id)
             .flex()
             .h(line)
@@ -157,11 +160,15 @@ impl RenderOnce for AnimatedNumber {
                             .into_any_element();
                     };
                     let start = old.to_digit(10).unwrap_or(0);
-                    let stack =
-                        div().flex().flex_col().children((0..10).map(|digit| {
-                            div().h(line).child(SharedString::from(digit.to_string()))
-                        }));
+                    // A block column lends the cell no baseline.
+                    let stack = div().children((0..10).map(|digit| {
+                        div()
+                            .debug_selector(move || format!("rolling-{digit}"))
+                            .h(line)
+                            .child(SharedString::from(digit.to_string()))
+                    }));
                     div()
+                        .debug_selector(|| "rolling-cell".into())
                         .h(line)
                         .overflow_hidden()
                         .child(stack.with_animation(