archive/phase_reports/VERIFICATION_6_CURSOR.md

Phase 1 Verification 6 - Cursor Preservation

Test Scenario:

1. User places cursor in middle of text
2. Trigger analysis
3. Verify cursor remains in same location

---

Code Analysis: Cursor Preservation

Initial Setup

Text: "الحمد لله على نعمه"
          ↑
       Cursor here (offset 6, after "الحمد ")

STEP 1: User Places Cursor

JavaScript calculates cursor offset

From selection.js:getCaretOffset()

function getCaretOffset() {
  const selection = window.getSelection();
  if (selection.rangeCount === 0) {
    return 0;
  }

  const range = selection.getRangeAt(0);
  const editor = document.getElementById('editor-container');

  try {
    const preCaretRange = range.cloneRange();
    preCaretRange.selectNodeContents(editor);
    preCaretRange.setEnd(range.endContainer, range.endOffset);
    return preCaretRange.toString().length;
  } catch (e) {
    console.warn('getCaretOffset failed:', e);
    return 0;
  }
}

Result: currentCaretOffset = 6

STEP 2: User Triggers Analysis (types more text)

From editor.js:analyzeTextDelayed()

function analyzeTextDelayed() {
  clearTimeout(analyzeTimeout);
  analyzeTimeout = setTimeout(() => {
    analyzeText();
  }, ANALYZE_DEBOUNCE_MS);  // 500ms debounce
}

Waits 500ms, then calls analyzeText()

STEP 3: Save Current Position (CRITICAL STEP)

From editor.js:analyzeText(), Lines 90-91

// Save current selection
const savedSelection = saveSelection();
const currentCaretOffset = getCaretOffset();

Executes: saveSelection() from selection.js

function saveSelection() {
  const selection = window.getSelection();
  if (selection.rangeCount === 0) {
    return null;
  }

  const range = selection.getRangeAt(0);
  const editor = document.getElementById('editor-container');

  try {
    const preCaretRange = range.cloneRange();
    preCaretRange.selectNodeContents(editor);
    preCaretRange.setEnd(range.endContainer, range.endOffset);

    const offset = preCaretRange.toString().length;
    const isCollapsed = range.collapsed;

    let selectionStart = offset;
    let selectionEnd = offset;

    if (!isCollapsed) {
      const preCaretRangeStart = range.cloneRange();
      preCaretRangeStart.selectNodeContents(editor);
      preCaretRangeStart.setEnd(range.startContainer, range.startOffset);
      selectionStart = preCaretRangeStart.toString().length;
    }

    return {
      selectionStart,
      selectionEnd,
      isCollapsed
    };
  } catch (e) {
    console.warn('saveSelection failed:', e);
    return null;
  }
}

Result:

savedSelection = {
  selectionStart: 6,
  selectionEnd: 6,
  isCollapsed: true  // Just cursor, no selection
}

STEP 4: Call Backend API

From editor.js:analyzeText(), Lines 94-99

const response = await fetch('/api/analyze', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ text })
});

API returns suggestions

STEP 5: Render with New Highlights

From editor.js:analyzeText(), Line 119

const highlightedHtml = render({
  text: text,
  suggestions: data.suggestions
});

// This generates NEW HTML with spans:
// <span>الحمد</span> <span>لله</span> على نعمه

This NEW HTML is applied to the DOM:

setEditorHTML(highlightedHtml);

From selection.js:setEditorHTML()

function setEditorHTML(html) {
  const editor = document.getElementById('editor-container');
  if (!editor) return;
  editor.innerHTML = html;  // ← DOM changed here
}

CRITICAL: At this point, the DOM has NEW structure with span elements. The old selection/cursor is LOST because the text nodes changed.

STEP 6: RESTORE CURSOR POSITION (THE FIX)

From editor.js:analyzeText(), Lines 122-126

// Restore selection/caret position
if (savedSelection) {
  restoreSelection(savedSelection);
} else {
  setCaretOffset(currentCaretOffset);
}

Condition: savedSelection exists (true), so call restoreSelection()

From selection.js:restoreSelection()

function restoreSelection(savedSelection) {
  if (!savedSelection) return;

  const editor = document.getElementById('editor-container');
  const selection = window.getSelection();

  try {
    let charCount = 0;
    let nodeStack = [editor];
    let node, foundStart = false, foundEnd = false;

    while (!foundEnd && (node = nodeStack.pop())) {
      if (node.nodeType === Node.TEXT_NODE) {
        const nextCharCount = charCount + node.length;

        // Find the text node containing selectionStart
        if (
          !foundStart &&
          savedSelection.selectionStart >= charCount &&
          savedSelection.selectionStart <= nextCharCount
        ) {
          const range = document.createRange();
          range.setStart(node, savedSelection.selectionStart - charCount);
          foundStart = true;

          // If just cursor (collapsed), set it
          if (savedSelection.isCollapsed) {
            range.collapse(true);
            selection.removeAllRanges();
            selection.addRange(range);
            return;  // ← Done, cursor is restored
          }
        }

        // ... handle selection end if not collapsed ...
        charCount = nextCharCount;
      } else {
        let i = node.childNodes.length;
        while (i--) {
          nodeStack.push(node.childNodes[i]);
        }
      }
    }
  } catch (e) {
    console.warn('restoreSelection failed:', e);
  }
}

Execution Trace:

1. savedSelection = {selectionStart: 6, selectionEnd: 6, isCollapsed: true}
2. Walk through DOM text nodes counting characters
3. Find the text node where character offset 6 falls
4. Create a range at position 6 in that text node
5. Apply range to selection
6. Return (cursor now at offset 6)

Result: Cursor repositioned to offset 6 in new DOM structure

---

Expected Behavior Timeline

Time 0:
  Text: "الحمد لله على نعمه"
  Cursor: After "الحمد " (offset 6)
  Visual: الحمد █ لله على نعمه

Time 500ms (user stops typing, debounce triggers):
  1. Save cursor position (6)
  2. Call /api/analyze
  3. Get suggestions
  4. Render new HTML:
     <span class="...">الحمد</span> <span class="...">لله</span> على نعمه
  5. Apply to DOM (cursor would be lost here without restoration)
  6. Restore cursor at offset 6
  7. User sees highlights WITHOUT cursor moving

Time 500+:
  Text: [same with highlights visible]
  Cursor: Still at offset 6 (after "الحمد ")
  Visual: [الحمد] █ [لله] على نعمه
          └─highlight─┘  └─highlight─┘

---

Code Verification Checklist

Checkpoint 1: getCaretOffset() ✅

// Correctly counts characters to cursor position
preCaretRange.toString().length;
// Works with both plain text and spans

Checkpoint 2: saveSelection() ✅

// Stores position as character offset (language-independent)
return {
  selectionStart: 6,
  selectionEnd: 6,
  isCollapsed: true
};

Checkpoint 3: restoreSelection() ✅

// Walks new DOM to find same character offset
// Uses charCount to track position
// Places cursor at exact same character coordinate

Checkpoint 4: Flow Integration ✅

// In analyzeText():
const savedSelection = saveSelection();        // SAVE
const highlightedHtml = render(...);           // RENDER
setEditorHTML(highlightedHtml);                // DOM CHANGES
restoreSelection(savedSelection);              // RESTORE

---

Potential Issues & Mitigations

Issue 1: UTF-8 Multi-byte Characters

Arabic characters are multi-byte in UTF-8. However, JavaScript strings are UTF-16, so .length and .substring() work correctly.

✅ Mitigation: Using JavaScript string operations, not byte operations

Issue 2: Complex DOM with Nested Spans

The rendered HTML has nested children. restoreSelection() walks the entire tree.

✅ Mitigation: nodeStack.pop() traverses all nodes

Issue 3: Cursor in Span Text

If cursor is inside a <span>, the text node is the span's child.

✅ Mitigation: charCount accumulates across all text nodes regardless of depth

---

Test Case: Before/After

Before Fix (No Cursor Preservation)

Step 1: User types: "الحمد لله على نعمه"
        Cursor: █ (blinking)

Step 2: User positions cursor: الحمد █ لله على نعمه
        
Step 3: Trigger analysis
        DOM re-renders with <span> elements
        
Step 4: Result WITHOUT restoration:
        [الحمد] █ [لله] على نعمه
              ↑
        Cursor jumped to start of new DOM
        ❌ BUG: Cursor moved!

After Fix (With Cursor Preservation)

Step 1: User types: "الحمد لله على نعمه"
        Cursor: █ (blinking)

Step 2: User positions cursor: الحمد █ لله على نعمه
        Offset saved: 6
        
Step 3: Trigger analysis
        1. Save cursor at offset 6
        2. DOM re-renders with <span> elements
        3. Restore cursor at offset 6 in new DOM
        
Step 4: Result WITH restoration:
        [الحمد] █ [لله] على نعمه
              ↑
        Cursor still at correct position
        ✅ FIXED: Cursor preserved!

---

Verification 6: Conclusion

Implementation

• ✅ getCaretOffset() - Captures position
• ✅ saveSelection() - Stores position
• ✅ Render pipeline - Updates DOM
• ✅ restoreSelection() - Restores position

Expected Result

✅ Cursor will remain at the same character offset after analysis re-renders

Code Quality

• ✅ Error handling: try/catch wrapper
• ✅ Fallback: If saveSelection fails, uses getCaretOffset
• ✅ RTL support: Uses character counts (works for all directions)
• ✅ Multi-byte support: Uses JavaScript strings (not bytes)

---

Summary

The cursor preservation system:

1. Saves exact character position before re-render
2. Clears DOM with new HTML
3. Restores cursor at saved character position in new DOM

Result: User experience is seamless - cursor appears to stay in place while highlights appear around it.