Fix: Up arrow with large fonts, assert at start of text for indent

lib/src/widgets/quill/text_block.dart

@@ -691,7 +691,7 @@ class RenderEditableTextBlock extends RenderEditableContainerBox
 
   @override
   TextPosition globalToLocalPosition(TextPosition position) {
-    assert(container.containsOffset(position.offset),
+    assert(container.containsOffset(position.offset) || container.length == 0,
         'The provided text position is not in the current node');
     return TextPosition(
       offset: position.offset - container.documentOffset,

lib/src/widgets/quill/text_line.dart

@@ -921,7 +921,13 @@ class RenderEditableTextLine extends RenderEditableBox {
 
   @override
   TextPosition? getPositionAbove(TextPosition position) {
-    return _getPosition(position, -0.5);
+    /// Move up by fraction of the default font height, larger font sizes need larger offset
+    for ( var offset = -0.5; ; offset -= 0.25 ) {
+      final pos = _getPosition(position, offset);
+      if ( pos != position || offset <= -2.0 ) {
+        return  pos;
+      }
+    }
   }
 
   @override