diff --git a/packages/carta-md/src/lib/internal/components/Input.svelte b/packages/carta-md/src/lib/internal/components/Input.svelte
index f6037a6a..87257f52 100644
--- a/packages/carta-md/src/lib/internal/components/Input.svelte
+++ b/packages/carta-md/src/lib/internal/components/Input.svelte
@@ -146,6 +146,11 @@
 				return { html, timestamp };
 			} catch (e) {
 				console.error(`Error executing speculative update: ${e}.`);
+				// The overlay still holds the previous text, but the debounced highlight is already
+				// on its way and will render `value`. Keep the baseline in sync with it, otherwise it
+				// stays at the pre-change text and the next patch sees the whole document as added,
+				// replaying it into the overlay (the text is duplicated until the debounce lands).
+				currentlyHighlightedValue = value;
 			}
 		}
 
diff --git a/packages/carta-md/src/lib/internal/speculative.ts b/packages/carta-md/src/lib/internal/speculative.ts
index 5102f272..fd1f5a07 100644
--- a/packages/carta-md/src/lib/internal/speculative.ts
+++ b/packages/carta-md/src/lib/internal/speculative.ts
@@ -223,7 +223,12 @@ function checkPosition(position: Position, lines: Element[]): Position {
 	if (nextPosition.span >= lineElement.children.length) {
 		nextPosition.char = 0;
 		nextPosition.span = 0;
-		nextPosition.line = line + 1;
+		// Only advance to the next line if there is one. The overlay of an empty document is a
+		// single line without spans (`<span class="line"></span>`), so wrapping past it would
+		// leave `line` at `lines.length`; writing there reads `children` of `undefined`.
+		if (line + 1 < lines.length) {
+			nextPosition.line = line + 1;
+		}
 	}
 
 	return nextPosition;