diff --git a/src/app/debug_log.rs b/src/app/debug_log.rs
index 1314994..35fbb5b 100644
--- a/src/app/debug_log.rs
+++ b/src/app/debug_log.rs
@@ -231,15 +231,36 @@ mod tests {
             let _ = std::fs::create_dir_all(parent);
         }
 
-        {
-            let mut f = std::fs::File::create(&path).expect("create oversized log");
-            std::io::Write::write_all(&mut f, &vec![b'X'; (MAX_LOG_SIZE_BYTES + 1) as usize])
-                .expect("write oversized log");
-        }
+        // The truncation guard only fires on a *freshly opened* file (or every
+        // CHECK_INTERVAL writes), so each attempt re-creates the oversized file
+        // and re-opens via `reset_for_test()` to genuinely exercise truncation.
+        // Creating it once before the loop would only test the first iteration:
+        // after any `log()` truncates, the file is small and later attempts just
+        // append, no longer exercising the truncation path.
+        //
+        // Cross-test interference is expected and the retry loop MUST NOT be
+        // collapsed into a single attempt. Production `log()` relies on
+        // PROCESS-GLOBAL statics: `CHECK_COUNTER` (AtomicU32), `LOG_FILE` (Mutex)
+        // and the `TEST_CACHE_HOME` redirect. `TEST_MUTEX` only serializes the two
+        // debug_log tests against each other; it does NOT exclude the hundreds of
+        // other tests on parallel threads that invoke the production
+        // `debug_log!`/`log()` macro. Once this test installs its tempdir into the
+        // process-global `TEST_CACHE_HOME`, those concurrent `log()` calls bump the
+        // shared `CHECK_COUNTER` and write to this test's tempdir, perturbing the
+        // size dance below. The retry loop tolerates that until one cycle wins.
+        //
+        // TODO(tech-debt): the principled fix is a per-test redirect gate that
+        // production `log()` consults first, isolating these globals; tracked as
+        // follow-up. Until then the test is empirically — not provably — robust.
 
+        // Empirically sufficient under N-way parallel `cargo test`; ~250ms worst
+        // case (RETRY_BUDGET iterations * the 5ms back-off below).
+        const RETRY_BUDGET: usize = 50;
         let mut truncated = false;
-        for attempt in 0..20 {
+        for attempt in 0..RETRY_BUDGET {
             reset_for_test();
+            std::fs::write(&path, vec![b'X'; (MAX_LOG_SIZE_BYTES + 1) as usize])
+                .expect("write oversized log");
             log(format_args!("attempt {attempt}"));
             let len = std::fs::metadata(&path).map(|m| m.len()).unwrap_or(0);
             if len > 0 && len < MAX_LOG_SIZE_BYTES {
@@ -258,7 +279,7 @@ mod tests {
         reset_for_test();
         assert!(
             truncated,
-            "log() never acquired mutex to truncate oversized file after 20 retries"
+            "log() never truncated the oversized file within {RETRY_BUDGET} retries"
         );
     }
 }
diff --git a/src/ui/viewer/tests.rs b/src/ui/viewer/tests.rs
index c931ef7..3394c8d 100644
--- a/src/ui/viewer/tests.rs
+++ b/src/ui/viewer/tests.rs
@@ -19,7 +19,6 @@ use std::thread;
 use tempfile::NamedTempFile;
 
 const DEFAULT_PAGE_HEIGHT: usize = 20;
-const TEST_CHANNEL_TIMEOUT_SECS: u64 = 1;
 
 fn create_test_file(content: &str) -> NamedTempFile {
     let mut file = NamedTempFile::new().unwrap();
@@ -89,9 +88,15 @@ fn test_viewer_loader_drop_cancels_worker() {
     drop(loader);
 
     assert!(cancel.load(Ordering::Relaxed));
+    // Drop stores `cancel` with `Release`; the worker loads it `Relaxed` in a
+    // `yield_now` loop, which still guarantees eventual visibility of the flag,
+    // so the worker is guaranteed to send. The generous 30s timeout (vs the old
+    // 1s) tolerates CPU starvation of the detached worker under heavy load, yet
+    // still bounds a genuine deadlock so CI fails fast instead of hanging the
+    // whole test binary.
     done_rx
-        .recv_timeout(std::time::Duration::from_secs(TEST_CHANNEL_TIMEOUT_SECS))
-        .unwrap();
+        .recv_timeout(std::time::Duration::from_secs(30))
+        .expect("worker should send done after observing cancel");
 }
 
 #[test]