List of steps to reproduce (step by step, including full links if applicable):

• Use Firefox - 91.0 (64-bit) Mozilla Firefox for Ubuntu canonical - 1.0
• Go to a Wikidata Lexeme, e.g., https://www.wikidata.org/wiki/Lexeme:L584516
• Add a form and copy-paste a long word to the input field (class="representation-widget_representation-value-input")
• Append a character to the representation in the input field

What happens?:

• It is not possible to see the appended character.
• It is not possible to scroll to the end of the string with the arrow keys.

What should have happened instead?:

• There should be horizontal scrolling
• The arrow keys should be possible to use to scroll to the end of the string

Further notes:

• The problem does not appear in the Chromium webbrowser.
• Adding a second character in Firefox makes it well-behaved where it is possible to see the character as well as horizonally scroll back and forth in the string.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc:

• Firefox - 91.0 (64-bit) Mozilla Firefox for Ubuntu canonical - 1.0
• Wikidata.org as per 16 August 2021 MediaWiki 1.37.0-wmf.18