const LimitedTextbox = fabric.util.createClass(fabric.Textbox, {
  onInput: function (e: any) {
    const oldText = this.text;

    /**
     * Sometimes, when you insert a line break, it doesn't appear to Fabric as an 'insertLineBreak' but simply sets the date to null.
     * Therefore, it's important to check that it's not deleting any characters, and if the date is null, it means that a new line is being inserted
     */
    if (e.inputType === 'insertLineBreak' || (e.inputType !== 'deleteContentBackward' && !e.data)) {
      this._textBeforeEdit = oldText;
      this.hiddenTextarea.value = oldText;
      this.text = oldText;
      return;
    } else if (e.inputType !== 'deleteContentBackward') {
      if (this.width > this.maxWidth) {
        this._textBeforeEdit = oldText;
        this.hiddenTextarea.value = oldText;
        this.text = oldText;

        // If you want to enable scaling to keep writing, there are issues when you delete a portion of text and not just a single character
        //
        // this.fontSizeList = [...(this.fontSizeList || []), this.fontSize];
        // this.fontSize *= this.maxWidth / (this.width + 1);
        // this.width = this.maxWidth;
        return;
      }

      if (this._textLines.length > this.maxLines) {
        this._textBeforeEdit = oldText;
        this.hiddenTextarea.value = oldText;
        this.text = oldText;
        return;
      }
    } else {
      // If you want to enable scaling to keep writing, there are issues when you delete a portion of text and not just a single character
      //
      // const lastFontSize = this.fontSizeList?.pop();
      // if (lastFontSize) {
      //   this.fontSize = lastFontSize;
      // }

      /**
       * It's needed because otherwise, if I write many characters, he automatically adjusts the width even if the visible text doesn't change.
       * So, every time I delete something, I reset the width.
       * I do it during deletion; otherwise, if I do it in the maxWidth check, I never block it, and it would keep writing
       */
      this.width = this.maxWidth;
    }

    this.callSuper('onInput', e);
  },
});

...

textbox = new LimitedTextbox('custom text', {
  maxLines: 1,
  maxWidth: 100,
});