Announce a month at the next row it opens, not only if it begins one

A heading was drawn only on a cell that both began a month and began a
row, so at seven columns most months were never named, and the one
heading on screen — always on the window's first cell — was wrong about
every row below it. Worse, two headings drawn on the same cell overprinted
each other. Now a row carries a heading whenever its first cell's month is
not the one last announced: a month starting mid-row is named on the next
row it opens, one row late and right about everything under it.

ui/dr-ui/src/library_ui.rs

@@ -2824,38 +2824,17 @@ fn load_window(window: &AppWindow, ctl: &Rc<LibraryController>) {
     // Month headings. The grid is ordered by capture time, so without these a
     // wall of thumbnails gives no sense of *when* you are looking — the
     // sidebar says it, but only if you consult it.
-    //
-    // Marked on the cell that both begins a month and begins a row: a heading
-    // stranded mid-row would label the cells to its left, which belong to the
-    // previous month.
     let columns = window.get_library_columns().max(1) as usize;
-    let mut previous_month: Option<(i64, i64)> = None;
-    let headings: Vec<String> = cells
-        .iter()
-        .enumerate()
-        .map(|(i, c)| {
-            let Some(t) = c.captured_at else {
-                return String::new();
-            };
+    let headings = period_headings(
+        cells.iter().map(|c| {
+            c.captured_at.map(|t| {
                 let (y, m, _, _) = civil_from_unix(t);
-            let is_new = previous_month != Some((y, m));
-            previous_month = Some((y, m));
-
-            // A heading is drawn above its row, so it can only sit on a cell
-            // that begins one — a heading stranded mid-row would appear to
-            // label the cells to its left, which belong to the month before.
-            //
-            // The first cell of the window always carries one, whichever
-            // column it lands in: a scrolled window would otherwise show no
-            // date at all until the next month began.
-            let begins_row = (i + offset) % columns == 0;
-            if i == 0 || (is_new && begins_row) {
-                format!("{} {y}", month_name(m))
-            } else {
-                String::new()
-            }
+                (y, m)
             })
-        .collect();
+        }),
+        offset,
+        columns,
+    );
 
     // What the outgoing model is still holding — see [`hold_thumbnails`].
     let held = {
@@ -7151,6 +7130,46 @@ fn stop(slot: &RefCell<Option<slint::Timer>>) {
     }
 }
 
+/// The heading each cell of a window carries: a month name on some, nothing
+/// on most.
+///
+/// A heading is drawn above its row, so it can only sit on a cell that
+/// begins one — stranded mid-row it would appear to label the cells to its
+/// left, which belong to the month before. So a row carries a heading when
+/// its first cell's month is not the one last announced. A month that begins
+/// mid-row is therefore announced at the *next* row it opens, which is one
+/// row late and still right about every cell under it. It used to be
+/// announced only if it happened to begin a row, which at seven columns
+/// meant most months were never named at all and the one heading on screen
+/// was wrong about everything below its first row.
+///
+/// The first cell of the window always carries one, whichever column it
+/// lands in: a scrolled window would otherwise show no date at all until
+/// the next month began.
+fn period_headings(
+    months: impl Iterator<Item = Option<(i64, i64)>>,
+    offset: usize,
+    columns: usize,
+) -> Vec<String> {
+    let columns = columns.max(1);
+    let mut announced: Option<(i64, i64)> = None;
+    months
+        .enumerate()
+        .map(|(i, month)| {
+            let Some((y, m)) = month else {
+                return String::new();
+            };
+            let begins_row = (i + offset).is_multiple_of(columns);
+            if i == 0 || (begins_row && announced != Some((y, m))) {
+                announced = Some((y, m));
+                format!("{} {y}", month_name(m))
+            } else {
+                String::new()
+            }
+        })
+        .collect()
+}
+
 /// The line in the library header that says what is being catalogued.
 ///
 /// For an account with a user, the user and the chosen subtree, or the
@@ -7180,6 +7199,45 @@ fn library_root_label(account: &Account) -> String {
 mod tests {
     use super::*;
 
+    #[test]
+    fn a_month_that_begins_mid_row_is_still_announced() {
+        // Four columns; August starts in the second column of the second
+        // row and October in the third of the third. Neither begins a row,
+        // and both must still be named — on the next row each opens.
+        let mar = Some((2024, 3));
+        let aug = Some((2024, 8));
+        let oct = Some((2025, 10));
+        let months = [
+            mar, mar, mar, mar, //
+            mar, aug, aug, aug, //
+            aug, aug, oct, oct, //
+            oct,
+        ];
+        let h = period_headings(months.into_iter(), 0, 4);
+        assert_eq!(h[0], "March 2024");
+        assert_eq!(h[4], "", "still March at the top of row two");
+        assert_eq!(h[5], "", "never mid-row");
+        assert_eq!(h[8], "August 2024");
+        assert_eq!(h[12], "October 2025");
+        assert_eq!(h.iter().filter(|s| !s.is_empty()).count(), 3);
+    }
+
+    #[test]
+    fn a_scrolled_window_names_its_first_cell_wherever_it_lands() {
+        let jan = Some((2026, 1));
+        let feb = Some((2026, 2));
+        // Offset 2 into a 4-column grid: the window's first cell is in the
+        // third column, and the row after it begins a new month.
+        let h = period_headings([jan, jan, feb, feb, feb].into_iter(), 2, 4);
+        assert_eq!(h[0], "January 2026");
+        assert_eq!(h[2], "February 2026", "cell 2 begins a row at offset 2");
+        assert_eq!(h[3], "");
+        // Undated cells carry nothing and announce nothing.
+        let h = period_headings([None, jan, None, None, jan].into_iter(), 0, 4);
+        assert_eq!(h[0], "");
+        assert_eq!(h[4], "January 2026");
+    }
+
     #[test]
     fn the_header_names_a_folder_library_by_its_folder() {
         let mut folder = Account::new("folder", "/var/tmp/dr-demo/library");