Bug 1665068 - Preserve layers for inactive GV windows. r=jnicol,snorp

Differential Revision: https://phabricator.services.mozilla.com/D93816

mobile/android/chrome/geckoview/geckoview.js

@@ -80,6 +80,11 @@ var ModuleManager = {
     // as docShell should be active by default, but this is not currently the
     // case so we force it here.
     aBrowser.docShellIsActive = true;
+    // By default all layers are discarded when a browser is set to inactive.
+    // GeckoView by default sets browsers to inactive every time they're not
+    // visible. To avoid flickering when changing tabs, we preserve layers for
+    // all loaded tabs.
+    aBrowser.preserveLayers(true);
 
     WindowEventDispatcher.registerListener(this, [
       "GeckoView:UpdateModuleState",

mobile/android/geckoview/src/androidTest/java/org/mozilla/geckoview/test/ContentDelegateTest.kt

@@ -260,22 +260,6 @@ class ContentDelegateTest : BaseSessionTest() {
         })
     }
 
-    @WithDisplay(width = 10, height = 10)
-    @Test fun paintStatusReset() {
-        mainSession.loadTestPath(HELLO_HTML_PATH)
-        sessionRule.waitUntilCalled(object : Callbacks.ContentDelegate {
-            @AssertCalled(count = 1)
-            override fun onFirstContentfulPaint(session: GeckoSession) {
-            }
-        })
-        mainSession.setActive(false)
-        sessionRule.waitUntilCalled(object : Callbacks.ContentDelegate {
-            @AssertCalled(count = 1)
-            override fun onPaintStatusReset(session: GeckoSession) {
-            }
-        })
-    }
-
     @Test fun webAppManifestPref() {
         val initialState = sessionRule.runtime.settings.getWebManifestEnabled()
         val jsToRun = "document.querySelector('link[rel=manifest]').relList.supports('manifest');"

mobile/android/geckoview/src/androidTest/java/org/mozilla/geckoview/test/ScreenshotTest.kt

@@ -156,7 +156,7 @@ class ScreenshotTest : BaseSessionTest() {
 
     @WithDisplay(height = SCREEN_HEIGHT, width = SCREEN_WIDTH)
     @Test
-    fun capturePixelsSessionDeactivatedActivated() {
+    fun capturePixelsWhileSessionDeactivated() {
         val screenshotFile = getComparisonScreenshot(SCREEN_WIDTH, SCREEN_HEIGHT)
 
         sessionRule.session.loadTestPath(COLORS_HTML_PATH)
@@ -167,18 +167,13 @@ class ScreenshotTest : BaseSessionTest() {
         })
 
         sessionRule.session.setActive(false)
-        sessionRule.waitUntilCalled(object : Callbacks.ContentDelegate {
-            @AssertCalled(count = 1)
-            override fun onPaintStatusReset(session: GeckoSession) {
-            }
-        })
 
-         sessionRule.session.setActive(true)
-         sessionRule.waitUntilCalled(object : Callbacks.ContentDelegate {
-             @AssertCalled(count = 1)
-             override fun onFirstContentfulPaint(session: GeckoSession) {
-             }
-         })
+        // Deactivating the session should trigger a flush state change
+        sessionRule.waitUntilCalled(object : Callbacks.ProgressDelegate {
+            @AssertCalled(count = 1)
+            override fun onSessionStateChange(session: GeckoSession,
+                                              sessionState: GeckoSession.SessionState) {}
+        })
 
         sessionRule.display?.let {
             assertScreenshotResult(it.capturePixels(), screenshotFile)