feat: add scroll offset for stack view in dev sample

samples/dev/index.js

@@ -20,6 +20,7 @@ const groups = {
 let stackCursor = null;
 let isMovingStackItem = false;
 let stackSnapshotBeforeMove = null;
+let stackViewOffset = 0;
 
 function labelFor(command) {
   return ({ add: '+', sub: '−', mul: '×', div: '÷', pow: 'y^x', recip: '1/x', sqr: 'x²' }[command] || command);
@@ -39,14 +40,14 @@ function getStackValue(index) {
   return calc.isValidIndex(index) ? calc.stack[index] : undefined;
 }
 
-function getLineValue(line) {
+function getDisplayValue(index) {
   if (calc.isEditing) {
-    if (line === 0) {
+    if (index === 0) {
       return calc.inputValue;
     }
-    return getStackValue(line - 1);
+    return getStackValue(index - 1);
   }
-  return getStackValue(line);
+  return getStackValue(index);
 }
 
 function hasStackSelection() {
@@ -57,6 +58,7 @@ function clearStackSelection() {
   stackCursor = null;
   isMovingStackItem = false;
   stackSnapshotBeforeMove = null;
+  stackViewOffset = 0;
 }
 
 function ensureValidSelection() {
@@ -140,16 +142,48 @@ function moveStackItem(direction) {
   stackCursor = targetIndex;
 }
 
+function getVisibleStackIndex(visualLine) {
+  return stackViewOffset + visualLine;
+}
+
+function clampStackViewOffset() {
+  const maxOffset = Math.max(0, calc.stack.length - 4);
+  if (stackViewOffset < 0) {
+    stackViewOffset = 0;
+  } else if (stackViewOffset > maxOffset) {
+    stackViewOffset = maxOffset;
+  }
+}
+
+function ensureSelectionVisible() {
+  if (!hasStackSelection()) {
+    stackViewOffset = 0;
+    return;
+  }
+
+  if (stackCursor < stackViewOffset) {
+    stackViewOffset = stackCursor;
+  } else if (stackCursor > stackViewOffset + 3) {
+    stackViewOffset = stackCursor - 3;
+  }
+
+  clampStackViewOffset();
+}
+
 function render() {
   const names = ['T', 'Z', 'Y', 'X'];
   const lines = [];
   const showStackIndexes = hasStackSelection() || isMovingStackItem;
 
-  for (let line = 3; line >= 0; line -= 1) {
-    const value = getLineValue(line);
-    const isSelected = stackCursor === line;
+  clampStackViewOffset();
+  ensureSelectionVisible();
+
+  for (let visualLine = 3; visualLine >= 0; visualLine -= 1) {
+    const stackIndex = getVisibleStackIndex(visualLine);
+    const value = getDisplayValue(stackIndex);
+    const isSelected = stackCursor === stackIndex;
     const classes = ['stack-line'];
-    const label = showStackIndexes ? String(line) : names[3 - line];
+    const label = showStackIndexes ? String(stackIndex) : names[3 - visualLine];
 
     if (isSelected) {
       classes.push(isMovingStackItem ? 'moving' : 'selected');