refactor(core): use Capacitor safe-area CSS variables on older WebViews (#30866)

Issue number: internal

---------

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

The safe area variables are only reliant on `env` variables that are
provided by devices.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

Capacitor 8 has released [safe area variable
fallbacks](https://capacitorjs.com/docs/apis/system-bars#android-note)
to provide consistent behaviors with older Android devices:

> Due to a [bug](https://issues.chromium.org/issues/40699457) in some
older versions of Android WebView (< 140), correct safe area values are
not available via the safe-area-inset-x CSS env variables. This plugin
will inject the correct inset values into a new CSS variable(s) named
--safe-area-inset-x that you can use as a fallback in your frontend
styles.

- Updated safe area variables to use the fallbacks provided by
Capacitor.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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  2. Update the BREAKING.md file with the breaking change.
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## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

Changes to the [core
file](2ee52d77c8/core/src/css/core.scss (L253-L260))
will be done on `main`.

---------

Co-authored-by: Brandy Smith <6577830+brandyscarney@users.noreply.github.com>
Co-authored-by: ionitron <hi@ionicframework.com>

core/src/components/app/test/safe-area/app.e2e.ts

@@ -5,7 +5,7 @@ import type { E2EPage } from '@utils/test/playwright';
 /**
  * Safe area tests only check top and bottom edges. RTL checks are not required here.
  */
-configs({ directions: ['ltr'] }).forEach(({ config, title, screenshot }) => {
+configs({ directions: ['ltr'], modes: ['ios', 'md', 'ionic-md'] }).forEach(({ config, title, screenshot }) => {
   test.describe(title('app: safe-area'), () => {
     const testOverlay = async (page: E2EPage, trigger: string, event: string, screenshotModifier: string) => {
       const presentEvent = await page.spyOnEvent(event);

core/src/css/ionic/core.ionic.scss

@@ -221,10 +221,12 @@ html.plt-ios.plt-pwa {
 
 @supports (padding-top: env(safe-area-inset-top)) {
   html {
-    --ion-safe-area-top: env(safe-area-inset-top);
-    --ion-safe-area-bottom: env(safe-area-inset-bottom);
-    --ion-safe-area-left: env(safe-area-inset-left);
-    --ion-safe-area-right: env(safe-area-inset-right);
+    // `--safe-area-inset-*` are set by Capacitor
+    // @see https://capacitorjs.com/docs/apis/system-bars#android-note
+    --ion-safe-area-top: var(--safe-area-inset-top, env(safe-area-inset-top));
+    --ion-safe-area-bottom: var(--safe-area-inset-bottom, env(safe-area-inset-bottom));
+    --ion-safe-area-left: var(--safe-area-inset-left, env(safe-area-inset-left));
+    --ion-safe-area-right: var(--safe-area-inset-right, env(safe-area-inset-right));
   }
 }