fix(mdviewer): cursor jump on formatBlock and edit-mode scroll sync

- Fix cursor jumping to previous line after applying heading format on
  empty lines: place cursor inside the newly created block element
- Block CM scroll-to-line when viewer has focus in edit mode to prevent
  highlight span creation from displacing the cursor

Author: abose

src-mdviewer/src/bridge.js

@@ -960,12 +960,17 @@ function handleScrollToLine(data) {
 
     // In edit mode, ignore scroll-based sync from CM to prevent feedback
     // loops (click in viewer → CM scroll → scroll sync back → viewer jumps).
-    // Only cursor-based sync (fromScroll=false) should reposition the viewer.
     if (fromScroll && getState().editMode) return;
 
     const viewer = document.getElementById("viewer-content");
     if (!viewer) return;
 
+    // In edit mode, skip CM cursor sync when the viewer has focus — the user
+    // is actively editing and highlight span creation/removal would displace
+    // the cursor.
+    if (getState().editMode && viewer.contains(document.activeElement)) return;
+    if (!viewer) return;
+
     const elements = viewer.querySelectorAll("[data-source-line]");
     let bestEl = null;
     let bestLine = -1;

src-mdviewer/src/components/editor.js

@@ -402,9 +402,33 @@ export function executeFormat(contentEl, command, value) {
         case "underline":
             document.execCommand(command, false, null);
             break;
-        case "formatBlock":
+        case "formatBlock": {
             document.execCommand("formatBlock", false, value);
+            // After formatBlock on an empty element, the browser may lose
+            // cursor position. Find the new block and place cursor inside it.
+            const sel2 = window.getSelection();
+            if (sel2 && sel2.rangeCount) {
+                let block = sel2.anchorNode;
+                if (block?.nodeType === Node.TEXT_NODE) block = block.parentElement;
+                // If cursor ended up outside the target block type, find it
+                const targetTag = value.replace(/[<>/]/g, "").toUpperCase();
+                if (block && block.tagName !== targetTag) {
+                    // Look for the newly created block near the cursor
+                    const allBlocks = contentEl.querySelectorAll(targetTag);
+                    for (const b of allBlocks) {
+                        if (b.textContent.trim() === "" || b.contains(sel2.anchorNode)) {
+                            const r = document.createRange();
+                            r.setStart(b, 0);
+                            r.collapse(true);
+                            sel2.removeAllRanges();
+                            sel2.addRange(r);
+                            break;
+                        }
+                    }
+                }
+            }
             break;
+        }
         case "createLink": {
             if (value) {
                 document.execCommand("createLink", false, value);