Keep a button at its label's width in a stack and a form

<x-stack> stretches its children across by default and <x-form> is a
one-track grid, so a button written directly in either drew as a
full-width pill. M3 keeps a button's width "dynamic to fit label" and
says not to "stretch buttons into long flat shapes on large windows".
A button there now sits at the start edge at its own width; a stack
aligned start, centre or end is untouched, and an application's own
width still wins (a cap on the button itself was rejected for exactly
that: it would have limited an app's inline-size, which CascadeTest
guards). Chromium test in both directions, failing without the rules.
Decided with the user (plan step 46).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/Browser/LayoutTest.php

@@ -72,6 +72,29 @@ it('spaces a stack with its token, never with its parent\'s, and hides it from a
         ->assertScript(layoutStyle('#hidden-from', 'display')." === 'none'");
 });
 
+it('keeps a button at its label\'s width in a stretched stack and a form, where the rest stretch', function () {
+    // M3: keep a button's width "dynamic to fit label"; don't "stretch buttons into long flat shapes
+    // on large windows". An application's own width still wins, and a centred stack centres it.
+    $body = '<style>.probe-full { inline-size: 100%; }</style>'
+        .'<x-stack id="stack" gap="space200" style="inline-size: 600px"><p id="stack-field">Name</p><x-button label="Save" id="stack-save" /><x-button label="Wide" class="probe-full" id="stack-wide" /></x-stack>'
+        .'<x-stack id="centred" align="center" style="inline-size: 600px"><x-button label="Save" id="centred-save" /></x-stack>'
+        .'<x-form id="form" style="inline-size: 600px"><x-input label="Name" /><x-button label="Go" id="form-go" /></x-form>';
+
+    layoutPage($body, 1280)
+        ->assertScript(layoutRect('#stack-field', 'width').' === 600')
+        ->assertScript(layoutRect('#stack-save', 'width').' < 200')
+        ->assertScript(layoutRect('#stack-save', 'left').' === '.layoutRect('#stack', 'left'))
+        ->assertScript(layoutRect('#stack-wide', 'width').' === 600')
+        ->assertScript('Math.abs(('.layoutRect('#centred-save', 'left').' + '.layoutRect('#centred-save', 'right').') / 2 - ('.layoutRect('#centred', 'left').' + '.layoutRect('#centred', 'right').') / 2) <= 1')
+        ->assertScript(layoutRect('#form-go', 'width').' < 200')
+        ->assertScript(layoutRect('#form-go', 'left').' === '.layoutRect('#form', 'left'));
+
+    // In a right-to-left document the start edge is the right.
+    layoutPage($body, 1280, 900, 'rtl')
+        ->assertScript(layoutRect('#stack-save', 'right').' === '.layoutRect('#stack', 'right'))
+        ->assertScript(layoutRect('#form-go', 'right').' === '.layoutRect('#form', 'right'));
+});
+
 it('stacks a row below its breakpoint and lays it side by side from it', function () {
     $body = '<x-row id="row" gap="space200" stack-below="medium"><span>One</span><span>Two</span></x-row>'
         .'<x-row id="aligned" align="start" stack-below="medium"><span>One</span></x-row>'

tests/Feature/Components/FieldTest.php

@@ -297,3 +297,9 @@ it('draws the file picker\'s button from its own stylesheet', function () {
         ->and((string) file_get_contents(__DIR__.'/../../../resources/css/components/field.css'))->not->toContain("[type='file']")
         ->and((string) file_get_contents(__DIR__.'/../../../resources/css/all.css'))->toContain("@import './components/file.css';");
 });
+
+it('keeps a button written among a form\'s fields at its label\'s width', function () {
+    // M3: keep a button's width "dynamic to fit label"; don't "stretch buttons into long flat shapes".
+    expect(ComponentStylesheet::read('form')->declarations('[data-md-form] > [data-md-button]'))
+        ->toBe(['justify-self' => 'start']);
+});

tests/Feature/Components/StackTest.php

@@ -41,3 +41,10 @@ it('draws the stack from a stylesheet in the layout layer, imported by all.css',
         ->toContain(":where([data-md-stack]) {\n        --md-gap: 0px;")
         ->and((string) file_get_contents(__DIR__.'/../../../resources/css/all.css'))->toContain("@import './layout/stack.css';");
 });
+
+it('keeps a button at its label\'s width while the stack stretches the rest', function () {
+    // M3: keep a button's width "dynamic to fit label"; don't "stretch buttons into long flat shapes".
+    $css = (string) file_get_contents(__DIR__.'/../../../resources/css/layout/stack.css');
+
+    expect($css)->toContain("[data-md-stack]:is(:not([data-md-align]), [data-md-align='stretch']) > [data-md-button] {\n        align-self: start;\n    }");
+});