fix(library): the wall takes a step off a single row, and a year of four frames stops stranding the shelf

docker/frontend/src/Library.tsx

@@ -4076,13 +4076,13 @@ function useWallWindow(count: number, on: boolean): { ref: RefObject<HTMLDivElem
       // run is a count the window feeds itself with. This is the same read the
       // settle effect above scrolls the wall by.
       const cols = Math.max(1, getComputedStyle(now).gridTemplateColumns.split(' ').length);
-      // The run the step below is read off. A run of a single row is no run at
-      // all — that is the last row of a shelf scrolled to its end — and a wall
-      // that has drawn nothing is nothing to read: the state's own first
-      // screenful stands until there is one.
+      // The run the step below is read off. A wall that has drawn nothing is
+      // nothing to read — the state's own first screenful stands — but a run of a
+      // single row is not nothing: it is the last row of a shelf scrolled to its
+      // end, or the whole wall of a shelf of four frames. It has no average, and
+      // the step it gets instead is its own height (below).
       const cards = now.querySelectorAll<HTMLElement>(':scope > .adm-card');
       const rowsDrawn = Math.floor(cards.length / cols);
-      if (rowsDrawn < 2) return;
       // The step from one row to the next, taken as the drawn run's own average.
       // The cards do NOT all stand the same height — a caption that wraps makes
       // its row taller — and `offsetTop` is rounded to whole pixels, so the
@@ -4098,15 +4098,31 @@ function useWallWindow(count: number, on: boolean): { ref: RefObject<HTMLDivElem
       // changes a card's own height (a caption wraps at the width it is given); a
       // resize that does not re-column keeps the step, and a spacer a few pixels out
       // is a scrollbar a few pixels out, not a blank screen.
-      if (colsRef.current !== cols) {
+      if (colsRef.current !== cols && rowsDrawn >= 2) {
         const drawn = (cards[(rowsDrawn - 1) * cols].getBoundingClientRect().top - cards[0].getBoundingClientRect().top) / (rowsDrawn - 1);
         if (drawn > 0) {
           colsRef.current = cols;
           pitchRef.current = drawn;
         }
       }
-      const pitch = pitchRef.current;
-      if (!(pitch > 0)) return;
+      // And the one-row run, which has no average to take: its own height is the
+      // step — one card plus the gap, the same fallback the settle effect takes.
+      // Returning here instead is a window that cannot grow, and there is a shelf
+      // it strands: the reset above puts a year of four frames at {0, 4}, one row
+      // of five columns, no average — and every pass after it reads that one row
+      // and leaves, so the shelf stands at four cards for the rest of the visit;
+      // another year, another folder, a reset, all four cards under a count of six
+      // hundred. The step is a guess, and a spacer a few pixels out is a scrollbar
+      // a few pixels out; a window left alone is the wall.
+      let pitch = pitchRef.current;
+      if (!(pitch > 0) || colsRef.current !== cols) {
+        const first = cards[0];
+        if (!first) return;
+        pitch = first.offsetHeight + WALL_GAP;
+        if (!(pitch > 0)) return;
+        colsRef.current = cols;
+        pitchRef.current = pitch;
+      }
       const rows = Math.ceil(count / cols);
       const row0 = Math.max(0, Math.floor(now.scrollTop / pitch) - WALL_ROWS);
       const row1 = Math.min(rows, Math.ceil((now.scrollTop + now.clientHeight) / pitch) + WALL_ROWS);