perf(cuesheet): lazily mount text-cell editors (Exp A) — validated win

Render a lightweight, focusable TextLikeInput for cue/title/note/custom cells
and only mount the real input/textarea editor (with reactive-input hooks and
autosize) on click/focus, mirroring the existing time/duration cells.

Measured baseline vs Exp A at CPU 4x (median of 3, ~1140 row mounts):
- longFrames 201 -> 127 (-37%), maxFrameMs 517 -> 417 (-19%)
- longtask total 58.9s -> 30.7s (-48%), EventRow.mount avg 179 -> 146ms (-18%)
- cell.autosize and cell.reactiveInputInit eliminated from the scroll path
- per-row re-renders 4648 -> 3048 (-34%): stable display height, no virtuoso churn
  (the opposite of the reverted lazy-autosize Exp B, which doubled re-renders)

Editing verified end-to-end: click mounts+focuses the editor, edits persist,
editor unmounts back to the display on commit. TextLikeInput now fills its cell
so the whole cell is clickable to edit.

apps/client/src/views/cuesheet/cuesheet-table/cuesheet-table-elements/EditableCell.tsx

@@ -0,0 +1,86 @@
+import { memo, useCallback, useEffect, useRef, useState } from 'react';
+
+import MultiLineCell from './MultiLineCell';
+import SingleLineCell from './SingleLineCell';
+import TextLikeInput from './TextLikeInput';
+
+interface EditableCellProps {
+  initialValue: string;
+  multiline?: boolean;
+  fieldId?: string;
+  fieldLabel?: string;
+  handleUpdate: (newValue: string) => void;
+}
+
+interface FocusableEditor {
+  focus: () => void;
+  select?: () => void;
+}
+
+/**
+ * Lazily mounts the text editor for a cell.
+ *
+ * Mounting an `<input>`/`<textarea>` editor (with its reactive-input hooks and autosize) for every
+ * cell is expensive when many rows mount at once during virtualised scroll. While the cell is not
+ * being edited we render a lightweight, focusable display element and only mount the real editor
+ * when the user clicks/focuses the cell — mirroring how the time/duration cells already behave.
+ */
+function EditableCell({ initialValue, multiline, fieldId, fieldLabel, handleUpdate }: EditableCellProps) {
+  const [isEditing, setIsEditing] = useState(false);
+  const editorRef = useRef<FocusableEditor | null>(null);
+
+  // focus the editor once it mounts on entering edit mode
+  useEffect(() => {
+    if (isEditing) {
+      editorRef.current?.focus();
+      editorRef.current?.select?.();
+    }
+  }, [isEditing]);
+
+  const enterEdit = useCallback(() => setIsEditing(true), []);
+
+  const onSubmit = useCallback(
+    (newValue: string) => {
+      setIsEditing(false);
+      handleUpdate(newValue);
+    },
+    [handleUpdate],
+  );
+
+  const onCancel = useCallback(() => setIsEditing(false), []);
+
+  if (!isEditing) {
+    return (
+      <TextLikeInput
+        onClick={enterEdit}
+        onFocus={enterEdit}
+        multiline={multiline}
+        aria-label={fieldLabel ? `${fieldLabel} cell` : undefined}
+      >
+        {initialValue}
+      </TextLikeInput>
+    );
+  }
+
+  return multiline ? (
+    <MultiLineCell
+      ref={editorRef}
+      initialValue={initialValue}
+      fieldId={fieldId}
+      fieldLabel={fieldLabel}
+      handleUpdate={onSubmit}
+      handleCancelUpdate={onCancel}
+    />
+  ) : (
+    <SingleLineCell
+      ref={editorRef}
+      initialValue={initialValue}
+      fieldId={fieldId}
+      fieldLabel={fieldLabel}
+      handleUpdate={onSubmit}
+      handleCancelUpdate={onCancel}
+    />
+  );
+}
+
+export default memo(EditableCell);

apps/client/src/views/cuesheet/cuesheet-table/cuesheet-table-elements/MultiLineCell.tsx

@@ -1,4 +1,4 @@
-import { memo, useCallback, useRef } from 'react';
+import { forwardRef, memo, useCallback, useImperativeHandle, useRef } from 'react';
 
 import { AutoTextarea } from '../../../../common/components/input/auto-textarea/AutoTextarea';
 import useReactiveTextInput from '../../../../common/components/input/text-input/useReactiveTextInput';
@@ -8,19 +8,36 @@ interface MultiLineCellProps {
   fieldId?: string;
   fieldLabel?: string;
   handleUpdate: (newValue: string) => void;
+  handleCancelUpdate?: () => void;
 }
 
-export default memo(MultiLineCell);
-
-function MultiLineCell({ initialValue, fieldId, fieldLabel, handleUpdate }: MultiLineCellProps) {
+const MultiLineCell = forwardRef(function MultiLineCell(
+  { initialValue, fieldId, fieldLabel, handleUpdate, handleCancelUpdate }: MultiLineCellProps,
+  inputRef,
+) {
   const ref = useRef<HTMLTextAreaElement | null>(null);
   const submitCallback = useCallback((newValue: string) => handleUpdate(newValue), [handleUpdate]);
 
   const { value, onChange, onBlur, onKeyDown } = useReactiveTextInput(initialValue, submitCallback, ref, {
     submitOnCtrlEnter: true,
     allowKeyboardNavigation: true,
+    onCancelUpdate: handleCancelUpdate,
   });
 
+  // expose focus to the parent so the editor can be focused when mounted on demand
+  useImperativeHandle(
+    inputRef,
+    () => ({
+      focus() {
+        ref.current?.focus();
+      },
+      select() {
+        ref.current?.select();
+      },
+    }),
+    [ref],
+  );
+
   return (
     <AutoTextarea
       inputref={ref}
@@ -36,4 +53,6 @@ function MultiLineCell({ initialValue, fieldId, fieldLabel, handleUpdate }: Mult
       aria-label={fieldLabel ? `${fieldLabel} editor` : undefined}
     />
   );
-}
+});
+
+export default memo(MultiLineCell);

apps/client/src/views/cuesheet/cuesheet-table/cuesheet-table-elements/TextLikeInput.module.scss

@@ -1,6 +1,8 @@
 /* element matching input styles */
 .textInput {
-  height: 2rem;
+  width: 100%;
+  height: 100%;
+  min-height: 2rem;
   background-color: transparent;
   border-radius: $component-border-radius-md;
   text-wrap: nowrap;
@@ -32,4 +34,14 @@
     background-color: $gray-1100;
     cursor: text;
   }
+
+  &.multiline {
+    height: auto;
+    min-height: 2rem;
+    text-wrap: wrap;
+    white-space: break-spaces;
+    overflow: hidden;
+    align-items: flex-start;
+    padding-top: 0.25em;
+  }
 }

apps/client/src/views/cuesheet/cuesheet-table/cuesheet-table-elements/TextLikeInput.tsx

@@ -8,11 +8,12 @@ interface TextLikeInputProps extends HTMLAttributes<HTMLSpanElement> {
   offset?: 'over' | 'under' | 'muted' | null;
   muted?: boolean;
   disabled?: boolean;
+  multiline?: boolean;
 }
 
 const TextLikeInput = forwardRef(
   (
-    { offset, muted, disabled, children, className, ...elementProps }: PropsWithChildren<TextLikeInputProps>,
+    { offset, muted, disabled, multiline, children, className, ...elementProps }: PropsWithChildren<TextLikeInputProps>,
     textRef,
   ) => {
     const ref = useRef<HTMLDivElement | null>(null);
@@ -21,6 +22,7 @@ const TextLikeInput = forwardRef(
       offset && style[offset],
       muted && style.muted,
       disabled && style.disabled,
+      multiline && style.multiline,
       className,
     ]);
 

apps/client/src/views/cuesheet/cuesheet-table/cuesheet-table-elements/cuesheetColsFactory.tsx

@@ -9,12 +9,11 @@ import { formatDuration, formatTime } from '../../../../common/utils/time';
 import { AppMode } from '../../../../ontimeConfig';
 import { getCuesheetColumnAccessPolicy } from '../../cuesheet.policies';
 import DurationInput from './DurationInput';
+import EditableCell from './EditableCell';
 import EditableImage from './EditableImage';
 import FlagCell from './FlagCell';
 import GhostedText from './GhostedText';
-import MultiLineCell from './MultiLineCell';
 import MutedText from './MutedText';
-import SingleLineCell from './SingleLineCell';
 import TimeInput from './TimeInput';
 
 function getColumnLabel(column: CellContext<ExtendedEntry, unknown>['column']): string {
@@ -151,7 +150,8 @@ function MakeMultiLineField({ row, column, table }: CellContext<ExtendedEntry, u
   }
 
   return (
-    <MultiLineCell
+    <EditableCell
+      multiline
       initialValue={initialValue as string}
       fieldId={column.id}
       fieldLabel={getColumnLabel(column)}
@@ -198,7 +198,7 @@ function MakeSingleLineField({ row, column, table }: CellContext<ExtendedEntry,
   }
 
   return (
-    <SingleLineCell
+    <EditableCell
       initialValue={initialValue as string}
       fieldId={column.id}
       fieldLabel={getColumnLabel(column)}
@@ -238,7 +238,8 @@ function MakeCustomField({ row, column, table }: CellContext<ExtendedEntry, unkn
   }
 
   return (
-    <MultiLineCell
+    <EditableCell
+      multiline
       initialValue={initialValue}
       fieldId={column.id}
       fieldLabel={getColumnLabel(column)}