diff --git a/packages/codemirror/src/extensions/custom_tab.ts b/packages/codemirror/src/extensions/custom_tab.ts
index c8ced1df2..3987827dd 100644
--- a/packages/codemirror/src/extensions/custom_tab.ts
+++ b/packages/codemirror/src/extensions/custom_tab.ts
@@ -24,19 +24,38 @@ const smartIndentWithTab: KeyBinding[] = [
 
             // Step 1: Handle non-empty selections → replace with tab
             if (selection.ranges.some(range => !range.empty)) {
-                for (let range of selection.ranges) {
-                    changes.push({ from: range.from, to: range.to, insert: "\t" });
-                    newSelections.push(EditorSelection.cursor(range.from + 1));
+                // If multiple lines are selected, insert a tab character at the start of each line
+                // and move the cursor to the position after the tab character.
+                const linesCovered = new Set<number>();
+                for (const range of selection.ranges) {
+                    const startLine = state.doc.lineAt(range.from);
+                    const endLine = state.doc.lineAt(range.to);
+
+                    for (let lineNumber = startLine.number; lineNumber <= endLine.number; lineNumber++) {
+                        linesCovered.add(lineNumber);
+                    }
+                }
+
+                if (linesCovered.size > 1) {
+                    // Multiple lines are selected, indent each line.
+                    return indentMore({ state, dispatch });
+                } else {
+                    // Single line selection, replace with tab.
+                    for (let range of selection.ranges) {
+                        changes.push({ from: range.from, to: range.to, insert: "\t" });
+                        newSelections.push(EditorSelection.cursor(range.from + 1));
+                    }
+
+                    dispatch(
+                        state.update({
+                            changes,
+                            selection: EditorSelection.create(newSelections),
+                            scrollIntoView: true,
+                            userEvent: "input"
+                        })
+                    );
                 }
 
-                dispatch(
-                    state.update({
-                        changes,
-                        selection: EditorSelection.create(newSelections),
-                        scrollIntoView: true,
-                        userEvent: "input"
-                    })
-                );
                 return true;
             }