fix: a file view states its pin once, so the axis holds still

`set_data_pin()` takes a range and `pin_event_window()` a pre/post
split; the file view handed the split to both. On a recording whose
stamps start before zero that stored a pin that no longer held the
data, so every second frame found the pin wrong and stated it in full
again -- rewriting the group's x range and discarding the user's
gesture with it. The view alternated between the file's span and the
corrupted one at the frame rate: a flicker in which no pan, zoom or
click survived. `[drop-probe]` now reports the view and the count of
pin statements, which must stay at two for a whole run.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ui.cpp

@@ -3392,11 +3392,19 @@ gboolean tick(gpointer user) {
       // made it a no-op. The held-event branch above has this shape for this reason: `set_data_pin()`
       // for the numbers, `pin_event_window()` when the view first becomes an event. A re-open into the
       // same row set with a different extent *is* a new event and re-states it.
-      const auto pin_view = [lo, hi](RowSet& rows) {
+      const auto pin_view = [lo, hi, app](RowSet& rows) {
+        // `set_data_pin()` takes the **range**, `pinned_window_is()`/`pin_event_window()` the
+        // pre/post split: passing the split to the former wrote the pin as `[lo, -lo]`-ish, which
+        // on a recording whose stamps start *before* zero is a window that no longer holds the
+        // data. The next frame's `pinned_window_is()` then read false, re-stated the pin, and the
+        // view alternated between the file's span and the corrupted one every frame -- a flicker
+        // with no interaction surviving it. A file whose first stamp was positive hid this: the
+        // corrupted pin was wider than the data and the clamp never fired.
         if (rows.pinned_window_is(-lo, hi)) {
-          rows.set_data_pin(-lo, hi);
+          rows.set_data_pin(lo, hi);
         } else {
           rows.pin_event_window(-lo, hi);
+          ++app->dbg_file_repins;
         }
         rows.set_trigger_marker(0.0, true);
       };