perf(calendar): re-render only the changed day cell, not the whole grid (#10)

Extracts the day-cell construction into a self-contained makeDayCell(d). Slot
changes and holiday/closure toggles now replace just that one cell
(cell.replaceWith) plus update export-button visibility, instead of rebuilding
the entire month grid and re-attaching every listener on each interaction.
Structural changes (month/year/slot-count) still do a full render.

Faithful extraction; verified headless that the page loads without JS errors
and tests pass. Interactive render coverage to follow in the test-layer issue.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/calendar.js

@@ -159,44 +159,13 @@ function renderGrid(wrapper, exportBtn) {
 
   const daysInMonth = new Date(currentYear, currentMonth, 0).getDate();
   const empNameById = Object.fromEntries(state.employees.map(e => [e.id, e.name]));
-  const entriesByDate = {};
-  for (const a of currentPlan) {
-    if (!entriesByDate[a.date]) entriesByDate[a.date] = [];
-    entriesByDate[a.date].push(a);
-  }
-  // Sort each date's entries alphabetically by employee name (stable slot order)
-  for (const date of Object.keys(entriesByDate)) {
-    entriesByDate[date].sort((x, y) =>
-      (empNameById[x.employeeId] ?? '').localeCompare(empNameById[y.employeeId] ?? '', 'de')
-    );
-  }
+  const updateExportVisibility = () => {
+    exportBtn.style.display = currentPlan.length > 0 ? 'inline-block' : 'none';
+  };
 
-  // Day-of-week header row
-  const headerGrid = document.createElement('div');
-  headerGrid.className = 'calendar-header-grid';
-  WD_LABELS.forEach(lbl => {
-    const cell = document.createElement('div');
-    cell.className = 'cal-header-cell';
-    cell.textContent = lbl;
-    headerGrid.appendChild(cell);
-  });
-  wrapper.appendChild(headerGrid);
-
-  // Body grid
-  const bodyGrid = document.createElement('div');
-  bodyGrid.className = 'calendar-body-grid';
-
-  // Offset empty cells before first day
-  const firstISO = currentYear + '-' + String(currentMonth).padStart(2,'0') + '-01';
-  const firstWD = (() => { const d = new Date(firstISO + 'T12:00:00Z').getUTCDay(); return d === 0 ? 7 : d; })();
-  for (let i = 1; i < firstWD; i++) {
-    const empty = document.createElement('div');
-    empty.className = 'cal-day empty-cell';
-    bodyGrid.appendChild(empty);
-  }
-
-  // Day cells
-  for (let d = 1; d <= daysInMonth; d++) {
+  // Build one day cell. Self-contained so a change to a single day re-renders
+  // just that cell (cell.replaceWith) instead of rebuilding the whole grid.
+  function makeDayCell(d) {
     const iso = currentYear + '-' + String(currentMonth).padStart(2,'0') + '-' + String(d).padStart(2,'0');
     const wdNum = (() => { const x = new Date(iso + 'T12:00:00Z').getUTCDay(); return x === 0 ? 7 : x; })();
     const isWeekend = wdNum >= 6;
@@ -209,7 +178,6 @@ function renderGrid(wrapper, exportBtn) {
     else if (isHoliday) cell.classList.add('holiday');
     else if (isClosure) cell.classList.add('closure');
 
-    // Day number + weekday label
     const dayNum = document.createElement('div');
     dayNum.className = 'day-num';
     dayNum.textContent = d + ' ';
@@ -219,9 +187,11 @@ function renderGrid(wrapper, exportBtn) {
     dayNum.appendChild(wdSpan);
     cell.appendChild(dayNum);
 
+    // Re-render only this cell after a change affecting this day
+    const refresh = () => { cell.replaceWith(makeDayCell(d)); updateExportVisibility(); };
+
     if (!isWeekend) {
       if (isHoliday || isClosure) {
-        // Show label + remove button
         const tag = document.createElement('div');
         tag.className = 'cal-day-tag-label';
         tag.textContent = isHoliday ? 'Feiertag' : 'Betriebsurlaub';
@@ -232,18 +202,18 @@ function renderGrid(wrapper, exportBtn) {
           if (isHoliday) toggleHoliday(state, iso);
           else toggleClosure(state, iso);
           saveState();
-          renderGrid(wrapper, exportBtn);
+          refresh();
         });
         tag.appendChild(removeBtn);
         cell.appendChild(tag);
       } else {
-        // N slots per day
         const slotCount = state.settings.moderatorsPerDay;
-        const dayEntries = entriesByDate[iso] ?? []; // already alphabetically sorted
+        const dayEntries = currentPlan
+          .filter(a => a.date === iso)
+          .sort((x, y) =>
+            (empNameById[x.employeeId] ?? '').localeCompare(empNameById[y.employeeId] ?? '', 'de'));
         const slotValues = []; // employeeId per slot index, '' if empty
-        for (let s = 0; s < slotCount; s++) {
-          slotValues.push(dayEntries[s]?.employeeId ?? '');
-        }
+        for (let s = 0; s < slotCount; s++) slotValues.push(dayEntries[s]?.employeeId ?? '');
 
         for (let slotIdx = 0; slotIdx < slotCount; slotIdx++) {
           const sel = document.createElement('select');
@@ -278,36 +248,64 @@ function renderGrid(wrapper, exportBtn) {
               currentPlan.push(entry);
             }
             saveState();
-            renderGrid(wrapper, exportBtn);
+            refresh();
           });
 
           cell.appendChild(sel);
         }
 
-        // FT / BU mark buttons
         const btnRow = document.createElement('div');
         btnRow.className = 'cal-btn-row';
 
         const ftBtn = document.createElement('button');
         ftBtn.type = 'button';
         ftBtn.textContent = 'Feiertag';
-        ftBtn.addEventListener('click', () => { toggleHoliday(state, iso); saveState(); renderGrid(wrapper, exportBtn); });
+        ftBtn.addEventListener('click', () => { toggleHoliday(state, iso); saveState(); refresh(); });
 
         const buBtn = document.createElement('button');
         buBtn.type = 'button';
         buBtn.textContent = 'Betriebsurl.';
-        buBtn.addEventListener('click', () => { toggleClosure(state, iso); saveState(); renderGrid(wrapper, exportBtn); });
+        buBtn.addEventListener('click', () => { toggleClosure(state, iso); saveState(); refresh(); });
 
         btnRow.append(ftBtn, buBtn);
         cell.appendChild(btnRow);
       }
     }
 
-    bodyGrid.appendChild(cell);
+    return cell;
+  }
+
+  // Day-of-week header row
+  const headerGrid = document.createElement('div');
+  headerGrid.className = 'calendar-header-grid';
+  WD_LABELS.forEach(lbl => {
+    const cell = document.createElement('div');
+    cell.className = 'cal-header-cell';
+    cell.textContent = lbl;
+    headerGrid.appendChild(cell);
+  });
+  wrapper.appendChild(headerGrid);
+
+  // Body grid
+  const bodyGrid = document.createElement('div');
+  bodyGrid.className = 'calendar-body-grid';
+
+  // Offset empty cells before first day
+  const firstISO = currentYear + '-' + String(currentMonth).padStart(2,'0') + '-01';
+  const firstWD = (() => { const d = new Date(firstISO + 'T12:00:00Z').getUTCDay(); return d === 0 ? 7 : d; })();
+  for (let i = 1; i < firstWD; i++) {
+    const empty = document.createElement('div');
+    empty.className = 'cal-day empty-cell';
+    bodyGrid.appendChild(empty);
+  }
+
+  // Day cells — each cell can re-render itself; no full-grid rebuild per click
+  for (let d = 1; d <= daysInMonth; d++) {
+    bodyGrid.appendChild(makeDayCell(d));
   }
 
   wrapper.appendChild(bodyGrid);
-  exportBtn.style.display = currentPlan.length > 0 ? 'inline-block' : 'none';
+  updateExportVisibility();
 }
 
 function renderSkippedWarning(box, skippedDates) {