fix: Keep the loading card's Cancel button still; deterministic Cancel test (#686)

The loading card was centred on its height, so Cancel moved when the progress bar or the saved preview appeared, and a click could miss it. That's what made the Cancel test flaky on slow runners. The card is now anchored at its top, Cancel sits in the heading row and the progress bar is always shown. Progress::hold_at lets the test hold a load at a feature boundary until it's cancelled.

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

crates/doc/src/progress.rs

@@ -10,11 +10,19 @@ use std::sync::{
 };
 
 /// Shared between the worker evaluating a design and the window showing its progress.
-#[derive(Debug, Default)]
+#[derive(Debug)]
 pub struct Progress {
     cancelled: AtomicBool,
     done: AtomicUsize,
     total: AtomicUsize,
+    /// The feature the evaluation waits at until cancelled ([`Progress::hold_at`]).
+    hold: AtomicUsize,
+}
+
+impl Default for Progress {
+    fn default() -> Self {
+        Progress { cancelled: AtomicBool::new(false), done: AtomicUsize::new(0), total: AtomicUsize::new(0), hold: AtomicUsize::new(usize::MAX) }
+    }
 }
 
 impl Progress {
@@ -25,6 +33,11 @@ impl Progress {
     pub fn is_cancelled(&self) -> bool {
         self.cancelled.load(Ordering::Relaxed)
     }
+    /// Make the watched evaluation wait before feature `index` until it is cancelled: tests
+    /// use it to act while a load is certainly still running, whatever the machine's speed.
+    pub fn hold_at(&self, index: usize) {
+        self.hold.store(index, Ordering::Relaxed);
+    }
     /// Features evaluated so far and the timeline's length (0 before evaluation starts).
     pub fn features(&self) -> (usize, usize) {
         (self.done.load(Ordering::Relaxed), self.total.load(Ordering::Relaxed))
@@ -85,6 +98,12 @@ impl Evaluation {
             let Some(cur) = c.as_ref() else { return false };
             if self.outer {
                 cur.progress.done.store(index, Ordering::Relaxed);
+                while index == cur.progress.hold.load(Ordering::Relaxed) && !cur.progress.is_cancelled() {
+                    #[cfg(not(target_arch = "wasm32"))]
+                    std::thread::sleep(std::time::Duration::from_millis(1));
+                    #[cfg(target_arch = "wasm32")]
+                    std::hint::spin_loop();
+                }
             }
             cur.progress.is_cancelled()
         })