fix: Correct modal focus behavior on macOS (#18995)

Fixes #18502

This PR changes the focus and blur events that we emit in Electron to listen to changes in key window rather than main window. It swaps out windowDidBecomeMain and windowDidResignMain for windowDidBecomeKey and windowDidResignKey, respectively.

shell/browser/ui/cocoa/atom_ns_window_delegate.mm

@@ -88,11 +88,11 @@ - (NSRect)windowWillUseStandardFrame:(NSWindow*)window
   return frame;
 }
 
-- (void)windowDidBecomeMain:(NSNotification*)notification {
+- (void)windowDidBecomeKey:(NSNotification*)notification {
   shell_->NotifyWindowFocus();
 }
 
-- (void)windowDidResignMain:(NSNotification*)notification {
+- (void)windowDidResignKey:(NSNotification*)notification {
   shell_->NotifyWindowBlur();
 }
 

spec/api-browser-window-spec.js

@@ -1637,6 +1637,34 @@ describe('BrowserWindow module', () => {
     })
   })
 
+  describe('focus event', () => {
+    it('should not emit if focusing on a main window with a modal open', (done) => {
+      const childWindowClosed = false
+      const child = new BrowserWindow({
+        parent: w,
+        modal: true,
+        show: false
+      })
+
+      child.once('ready-to-show', () => {
+        child.show()
+      })
+
+      child.on('show', () => {
+        w.once('focus', () => {
+          expect(child.isDestroyed()).to.equal(true)
+          done()
+        })
+        w.focus() // this should not trigger the above listener
+        child.close()
+      })
+
+      // act
+      child.loadURL(server.url)
+      w.show()
+    })
+  })
+
   describe('sheet-begin event', () => {
     let sheet = null