Fix password input button scaling

Fixes an issue where the password input toggle button would stretch to fill any space not used by the input, such as if the input has a restricted max width.

I think the `flex-grow` property may have been a leftover from earlier in the component's development, as it doesn't appear to be needed now.

Adds a review app example to help keep this in check in future.

CHANGELOG.md

@@ -11,6 +11,7 @@ We've made fixes to GOV.UK Frontend in the following pull requests:
 - [#4906: Update the icon in the warning text component to match the defined text colour and background colour, rather than always being white on black](https://github.com/alphagov/govuk-frontend/pull/4906)
 - [#4919: Use canvas colour for cookie banner over hardcoded grey](https://github.com/alphagov/govuk-frontend/pull/4919)
 - [#4899: Remove indents from conditional reveals in radios and checkboxes](https://github.com/alphagov/govuk-frontend/pull/4899)
+- [#4935: Fix password input button unexpectedly stretching](https://github.com/alphagov/govuk-frontend/pull/4935)
 
 ## GOV.UK Frontend v5.3.0 (Feature release)
 

packages/govuk-frontend/src/govuk/components/password-input/_index.scss

@@ -20,7 +20,7 @@
     // IE 11 and Microsoft Edge comes with its own password reveal function. We want to hide it,
     // so that there aren't two controls presented to the user that do the same thing but aren't in
     // sync with one another. This doesn't affect the function that allows Edge users to toggle
-    // password visibility by pressing Alt+F8, which cannot be programatically disabled.
+    // password visibility by pressing Alt+F8, which cannot be programmatically disabled.
     &::-ms-reveal {
       display: none;
     }
@@ -41,7 +41,6 @@
     @include govuk-media-query($from: mobile) {
       // Buttons are normally 100% width on this breakpoint, but we don't want that in this case
       width: auto;
-      flex-grow: 1;
       flex-shrink: 0;
       flex-basis: 5em;
 

packages/govuk-frontend/src/govuk/components/password-input/password-input.yaml

@@ -152,8 +152,15 @@ examples:
         isPageHeading: true
       id: password-input-with-page-heading
       name: test-name
+  - name: with input width class
+    options:
+      label:
+        text: Password
+      id: password-input-width
+      name: password
+      classes: govuk-input--width-10
   - name: with new-password autocomplete
-    description: Browsers and password managers should prompt to generate a password.
+    description: Supporting browsers and password managers should prompt to generate a password.
     options:
       label:
         text: Password