feat(PageHeader): Convert PageHeader to CSS modules behind feature flag

.changeset/friendly-tomatoes-yell.md

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+---
+"@primer/react": minor
+---
+
+Convert PageHeader to CSS modules behind feature flag

packages/react/src/PageHeader/PageHeader.module.css

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+.PageHeader {
+  /* Grid Row Order */
+  --grid-row-order-context-area: 1;
+  --grid-row-order-leading-action: 2;
+  --grid-row-order-breadcrumbs: 2;
+  --grid-row-order-title-area: 2;
+  --grid-row-order-trailing-action: 2;
+  --grid-row-order-actions: 2;
+  --grid-row-order-description: 3;
+  --grid-row-order-navigation: 4;
+
+  /* Title Area Region Order */
+  --title-area-region-order-leading-visual: 0;
+  --title-area-region-order-title: 1;
+  --title-area-region-order-trailing-visual: 2;
+
+  /* Context Area Region Order */
+  --context-area-region-order-parent-link: 0;
+  --context-area-region-order-context-bar: 1;
+  --context-area-region-order-context-area-actions: 2;
+
+  display: grid;
+
+  /* We have max 5 columns. */
+  grid-template-columns: auto auto auto auto 1fr;
+  grid-template-areas:
+    'context-area context-area context-area context-area context-area'
+    'leading-action breadcrumbs title-area trailing-action actions'
+    'description description description description description'
+    'navigation navigation navigation navigation navigation';
+
+  /* 
+    line-height is calculated with calc(height/font-size) and the below numbers are from @primer/primitives.
+    --custom-font-size, --custom-line-height, --custom-font-weight are custom properties that can be used to override the below values.
+    We don't want these values to be overridden but still want to allow consumers to override them if needed.
+  */
+  &:has([data-component='TitleArea'][data-size-variant='large']) {
+    font-size: var(--custom-font-size, var(--text-title-size-large, 2rem));
+    font-weight: var(--custom-font-weight, var(--base-text-weight-normal, 400));
+    line-height: var(--custom-line-height, var(--text-title-lineHeight-large, 1.5)); /* calc(48/32) */
+
+    --title-line-height: var(--custom-line-height, var(--text-title-lineHeight-large, 1.5));
+  }
+
+  &:has([data-component='TitleArea'][data-size-variant='medium']) {
+    font-size: var(--custom-font-size, var(--text-title-size-medium, 1.25rem));
+    font-weight: var(--custom-font-weight, var(--base-text-weight-semibold, 600));
+    line-height: var(--custom-line-height, var(--text-title-lineHeight-medium, 1.6)); /* calc(32/20) */
+
+    --title-line-height: var(--custom-line-height, var(--text-title-lineHeight-medium, 1.6));
+  }
+
+  &:has([data-component='TitleArea'][data-size-variant='subtitle']) {
+    font-size: var(--custom-font-size, var(--text-title-size-medium, 1.25rem));
+    font-weight: var(--custom-font-weight, var(--base-text-weight-normal, 400));
+    line-height: var(--custom-line-height, var(--text-title-lineHeight-medium, 1.6)); /* calc(32/20) */
+
+    --title-line-height: var(--custom-line-height, var(--text-title-lineHeight-medium, 1.6));
+  }
+
+  & [data-component='PH_LeadingAction'],
+  & [data-component='PH_TrailingAction'],
+  & [data-component='PH_Actions'],
+  & [data-component='PH_LeadingVisual'],
+  & [data-component='PH_TrailingVisual'] {
+    height: calc(var(--title-line-height) * 1em);
+  }
+
+  & [data-hidden-all] {
+    display: none;
+  }
+
+  & [data-hidden-narrow] {
+    @media screen and (max-width: 768px) {
+      display: none;
+    }
+  }
+
+  & [data-hidden-regular] {
+    @media screen and (min-width: 768px) {
+      display: none;
+    }
+  }
+
+  & [data-hidden-wide] {
+    @media screen and (min-width: 1440px) {
+      display: none;
+    }
+  }
+}
+
+.ContextArea {
+  display: flex;
+  padding-bottom: var(--base-size-8);
+  font-size: var(--text-body-size-medium, 0.875rem);
+  font-weight: var(--base-text-weight-light);
+  line-height: var(--text-body-lineHeight-medium, 1.4285);
+  flex-direction: row;
+  grid-row: var(--grid-row-order-context-area);
+  grid-area: context-area;
+  align-items: center;
+  gap: var(--stack-gap-condensed);
+}
+
+.ParentLink {
+  display: flex;
+  align-items: center;
+  order: var(--context-area-region-order-parent-link);
+  gap: var(--stack-gap-condensed);
+}
+
+.ContextBar {
+  display: flex;
+  order: var(--context-area-region-order-context-bar);
+}
+
+.ContextAreaActions {
+  display: flex;
+  flex-direction: row;
+  order: var(--context-area-region-order-context-area-actions);
+  align-items: center;
+  gap: var(--stack-gap-condensed);
+  flex-grow: 1;
+  justify-content: flex-end;
+}
+
+.TitleArea {
+  grid-row: var(--grid-row-order-title-area);
+  grid-area: title-area;
+  display: flex;
+  gap: var(--stack-gap-condensed);
+  flex-direction: row;
+  align-items: flex-start;
+}
+
+.LeadingAction {
+  display: flex;
+  padding-right: var(--base-size-8);
+  grid-row: var(--grid-row-order-leading-action);
+  grid-area: leading-action;
+  align-items: center;
+}
+
+.Breadcrumbs {
+  display: flex;
+  padding-right: var(--base-size-8);
+  font-size: var(--text-body-size-medium, 0.875rem);
+  font-weight: var(--base-text-weight-light);
+  line-height: var(--text-body-lineHeight-medium, 1.4285);
+  grid-row: var(--grid-row-order-breadcrumbs);
+  grid-area: breadcrumbs;
+  align-items: center;
+}
+
+.LeadingVisual {
+  /* using flex and order to display the leading visual in the title area. */
+  display: flex;
+  order: var(--title-area-region-order-leading-visual);
+  align-items: center;
+}
+
+.Title {
+  /* using flex and order to display the title in the title area. */
+  display: flex;
+  order: var(--title-area-region-order-title);
+  font-size: inherit;
+  font-weight: inherit;
+}
+
+.TrailingVisual {
+  /* using flex and order to display the trailing visual in the title area. */
+  display: flex;
+  order: var(--title-area-region-order-trailing-visual);
+  align-items: center;
+}
+
+.TrailingAction {
+  display: flex;
+  padding-left: var(--base-size-8);
+  grid-row: var(--grid-row-order-trailing-action);
+  grid-area: trailing-action;
+  align-items: center;
+}
+
+.Actions {
+  display: flex;
+  min-width: max-content;
+  padding-left: var(--base-size-8);
+  flex-direction: row;
+  grid-row: var(--grid-row-order-actions);
+  grid-area: actions;
+  gap: var(--stack-gap-condensed);
+  justify-content: flex-end;
+  align-items: center;
+}
+
+.Description {
+  display: flex;
+  padding-top: var(--base-size-8);
+  font-size: var(--text-body-size-medium, 0.875rem);
+  font-weight: var(--base-text-weight-light);
+  line-height: var(--text-body-lineHeight-medium, 1.4285);
+  flex-direction: row;
+  grid-row: var(--grid-row-order-description);
+  grid-area: description;
+  align-items: center;
+  gap: var(--stack-gap-condensed);
+}
+
+.Navigation {
+  display: block;
+  padding-top: var(--base-size-8);
+  font-size: var(--text-body-size-medium, 0.875rem);
+  font-weight: var(--base-text-weight-light);
+  line-height: var(--text-body-lineHeight-medium, 1.4285);
+  grid-row: var(--grid-row-order-navigation);
+  grid-area: navigation;
+}