Say where the grid is on the capture-time axis from the first frame

The sidebar's marker rested greyed at mid-track until the first scroll or
scrub. The reasoning was that anchoring it would imply a choice the user
had not made -- but that reads the marker as reporting an intention, and
it does not. The sidebar's whole claim is to say *when* you are, and that
is known from the first frame: the grid is at the top of the library, or
wherever it was last left.

So a launch opened with the marker halfway down an axis whose visible
photographs were all from the wrong end of it. Dimmed rather than absent,
which made it look like a reading rather than the absence of one.

Seeded in `refresh_timeline` -- the one place that decides what the
marker says, and the one that runs on every route which builds the axis
-- and only when nothing has claimed it, so a scroll or a scrub still
speaks for itself.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/library_ui.rs

@@ -225,8 +225,12 @@ pub struct LibraryController {
     /// held until it reaches a doubling. Without it a slow spread would either
     /// do nothing or, if each update were rounded, leap several levels.
     pinch_accum: RefCell<f32>,
-    /// The instant the grid is showing. `None` until the user has moved the
+    /// The instant the grid is showing.
-    /// timeline, which is what leaves the marker resting at the middle.
+    ///
+    /// `None` only before the first axis has been built: `refresh_timeline`
+    /// seeds it from wherever the view sits, so the marker reports a position
+    /// from the first frame rather than resting greyed at mid-track until the
+    /// user happens to scroll. A scroll or a scrub then sets it directly.
     current_bucket: RefCell<Option<i64>>,
     /// What the rating filter bar is narrowed to.
     ///
@@ -4182,6 +4186,29 @@ fn start_thumbnail_sweep(window: &AppWindow, ctl: &Rc<LibraryController>) {
 /// from the data rather than fixing it means the histogram is informative at
 /// both scales instead of one flat bar or ten thousand slivers.
 fn refresh_timeline(window: &AppWindow, catalog: &Catalog, ctl: &Rc<LibraryController>) {
+    // Where the grid sits, if nothing has said so yet.
+    //
+    // **The marker used to rest greyed at mid-track until the first scroll or
+    // scrub**, on the reasoning that anchoring it would imply a choice the user
+    // had not made. That reads the marker as reporting an intention, and it does
+    // not: the sidebar's whole claim is to say *when* you are, and the answer is
+    // known from the first frame — the grid is at the top of the library, or at
+    // whatever position was restored into it. A launch that opened with the
+    // marker halfway down an axis whose photographs were all from the wrong end
+    // was simply wrong, and dimmed rather than absent, which made it look like a
+    // reading rather than the absence of one.
+    //
+    // Seeded here rather than at the end of the scan because this is the one
+    // place that decides what the marker says, and it runs on every route that
+    // builds the axis. Only when nothing has claimed it: a scroll or a scrub
+    // sets it directly, and this must never overwrite them.
+    let unanchored = ctl.current_bucket.borrow().is_none();
+    if unanchored {
+        if let Some(when) = capture_time_at(ctl, catalog, ctl.resume_at.get()) {
+            *ctl.current_bucket.borrow_mut() = Some(when);
+        }
+    }
+
     // Scoped to whatever the grid is showing. A collection's histogram drawn
     // over the whole library's span said almost nothing: every bar for a
     // fortnight in Arosa landed in one column of a fifteen-year axis.
@@ -4574,9 +4601,10 @@ fn scrub_to(window: &AppWindow, ctl: &Rc<LibraryController>, when: i64) {
             .unwrap_or(0) as usize
     };
 
-    // Record where the grid now sits, which anchors the timeline marker. Until
+    // Record where the grid now sits, which anchors the timeline marker.
-    // the first scrub this stays `None` and the marker rests at the middle
+    // `refresh_timeline` seeds this from the view's own position when nothing
-    // rather than implying a choice the user has not made.
+    // has claimed it, so by the time a scrub arrives it is an update rather than
+    // the first word.
     *ctl.current_bucket.borrow_mut() = Some(when);
     *ctl.offset.borrow_mut() = position;
     // Move the viewport as well as the window. Cells are drawn at their

ui/dr-ui/ui/library.slint

@@ -93,9 +93,10 @@ export component Timeline inherits Rectangle {
     /// happened to fall — never under the pointer. Negative means "not
     /// anchored".
     in property <float> current-fraction: -1;
-    /// True once the user has taken control. Until then the marker rests at the
+    /// Whether the marker has a position to report. False only before the first
-    /// middle rather than pinning to either end, which would imply a selection
+    /// axis has been built — Rust seeds it from wherever the grid sits, so a
-    /// that has not been made.
+    /// launch draws the marker where the photographs on screen are rather than
+    /// leaving it dimmed at mid-track waiting for a scroll.
     in property <bool> anchored: false;
 
     /// Scrub to a fraction along the visible span. Rust turns it into an