editor: Fix issue with splitting at index zero

src/editor/editor.c

@@ -1132,8 +1132,35 @@ text_editor_split_at_mark (TextEditor *self,
 
     current = split->paragraph;
 
-    // Case 1: Split is happening on the last index
-    if (split->index == text_paragraph_get_length (current))
+    // Case 1: Split is happening on the first index
+    if (split->index == 0)
+    {
+        TextParagraph *temp;
+
+        // Move all runs in the current paragraph to a new paragraph
+        // after this one and make the current paragraph contain an
+        // empty run.
+
+        // In reality, we simply prepend a blank paragraph as that is
+        // faster.
+
+        new = text_paragraph_new ();
+        text_paragraph_append_run (new, text_run_new (""));
+
+        parent = text_node_get_parent (TEXT_NODE (current));
+        text_node_insert_child_before (parent, TEXT_NODE (new), TEXT_NODE (current));
+
+        // In order for mark distribution to work as expected, we need
+        // to pretend like we created a new paragraph after the current
+        // one and moved all runs to it. Hence we must swap the 'new' and
+        // 'current' variables.
+        temp = current;
+        current = new;
+        new = temp;
+    }
+
+    // Case 2: Split is happening on the last index
+    else if (split->index == text_paragraph_get_length (current))
     {
         // Append a new paragraph with empty run
         new = text_paragraph_new ();
@@ -1143,7 +1170,7 @@ text_editor_split_at_mark (TextEditor *self,
         text_node_insert_child_after (parent, TEXT_NODE (new), TEXT_NODE (current));
     }
 
-    // Case 2: Split happens mid-paragraph
+    // Case 3: Split happens mid-paragraph
     else
     {
         TextRun *start;
@@ -1209,7 +1236,7 @@ text_editor_split_at_mark (TextEditor *self,
 
         // Mark is on split point
         if (mark->paragraph == current &&
-              mark->index == split->index)
+            mark->index == split->index)
         {
             _distribute_mark (mark, current, split->index, new, 0);
             continue;