scale-dropdown-select: readonly prop has no effect

[popadicbranislav]

Scale Version

@telekom/scale-components@3.0.0-beta.160. The relevant code on main looks
unchanged, so I do not think 3.0.0-beta.161 fixes it either.

Framework and version

Vue 3.5.33, using the web components directly via defineCustomElements from
@telekom/scale-components/loader — no auto-generated wrapper package. The
behaviour is in the component itself, so the framework should not matter.

Current Behavior

scale-dropdown-select accepts a readonly prop — it is in the shipped type
declarations — but setting it does nothing. The dropdown still opens on click
and via the keyboard, and the value can still be changed. There is also no
visual difference between a read-only and an editable field.

disabled works correctly; only readonly is affected.

From reading the source, readonly looks unimplemented rather than broken:

• setOpen() early-returns on this.disabled only, so readonly never
  prevents the listbox from opening.
• The combobox trigger sets tabindex: this.disabled ? '-1' : '0', so a
  read-only field stays in the tab order.
• getBasePartMap() emits a readonly part token, but dropdown-select.css
  has no rule matching it, so there is no read-only styling.
• handleClearClick() is the only handler that checks readonly, and it is
  only reachable when allow-clear is set.

Expected Behavior

A read-only dropdown should not open and its value should not be changeable,
consistent with readonly on scale-text-field. Ideally it would also be
visually distinguishable from an editable field.

Code Reproduction

https://codesandbox.io/p/devbox/scale-components-plain-html-template-forked-wsz56r

No framework needed — this is enough:

<scale-dropdown-select readonly label="City" value="berlin">
  <scale-dropdown-select-item value="berlin">Berlin</scale-dropdown-select-item>
  <scale-dropdown-select-item value="bonn">Bonn</scale-dropdown-select-item>
</scale-dropdown-select>

1. Click the field — the listbox opens.
2. Pick "Bonn" — the value changes, despite readonly.
3. Press Tab — the field is still in the tab order, and
   Enter/ArrowDown opens it too.

Swapping readonly for disabled behaves as expected, which is the contrast
that makes this look like an oversight.

Desktop (please complete the following information):

• OS: Windows 11
• Browser: Chrome
• Version: -