chore(lint): forbid reading the display size from the mermaid source

`no-restricted-globals` rejects `screen` under `packages/mermaid/src`, so
rendered geometry cannot start depending on the size of the viewer's monitor
again.

Author: filipsajdak

eslint.config.js

@@ -232,6 +232,26 @@ export default tseslint.config(
       'tsdoc/syntax': 'error',
     },
   },
+  {
+    files: ['packages/mermaid/src/**/*.{ts,js}'],
+    rules: {
+      'no-restricted-globals': [
+        'error',
+        {
+          globals: [
+            {
+              name: 'screen',
+              message:
+                'Rendered geometry must not depend on the display size, or the same diagram lays out differently on different machines.',
+            },
+          ],
+          // Also rejects `window.screen`, `globalThis.screen` and `self.screen`, which
+          // the bare-name form lets through.
+          checkGlobalObject: true,
+        },
+      ],
+    },
+  },
   {
     files: ['**/*.spec.{ts,js}', 'cypress/**', 'demos/**', '**/docs/**'],
     rules: {

packages/mermaid/src/diagrams/c4/c4Renderer.spec.ts

@@ -82,6 +82,7 @@ describe('C4 grid placement', () => {
   afterEach(() => {
     // One test below reports a different display size. Drop that override so the rest
     // see the environment's own value and the results do not depend on test order.
+    // eslint-disable-next-line no-restricted-globals -- proving the layout ignores the display needs to set one
     delete (globalThis.screen as unknown as { availWidth?: number }).availWidth;
   });
 
@@ -102,6 +103,7 @@ describe('C4 grid placement', () => {
   it('places elements identically whatever the display reports', async () => {
     const code = `C4Context\n${systems(5)}`;
     const asSeenOn = async (availWidth: number, id: string) => {
+      // eslint-disable-next-line no-restricted-globals -- the point of this test is that the value is ignored
       Object.defineProperty(globalThis.screen, 'availWidth', {
         value: availWidth,
         configurable: true,