[gnome-shell/mcatanzaro/#734996] Delay login animation until after wallpaper has loaded

[Michael Catanzaro <mcatanzaro src gnome org>]

commit eeee79cf6119957b64d87e25b4c11e9586150059
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Mon Jul 20 19:00:32 2020 -0500

    Delay login animation until after wallpaper has loaded
    
    Currently, the login animation can occur before the user's wallpaper has
    been loaded. When this happens, we wind up displaying a solid blue
    background for half a second or so before the proper background is
    displayed. This looks jarring and bad. It's great that we can start
    GNOME quickly, but starting up before the wallpaper is ready is *too*
    quickly.
    
    I've been meaning to fix this since 2014. Better late than never! We can
    just have BackgroundManager emit a loaded signal the first time it loads
    its first background, and have the startup animation code wait for that
    before proceeding.
    
    Some of this code is by Florian, who helped with promisifying. Thanks!
    
    https://bugzilla.gnome.org/show_bug.cgi?id=734996

 js/ui/background.js |  9 +++++++++
 js/ui/layout.js     | 22 ++++++++++++++++++----
 2 files changed, 27 insertions(+), 4 deletions(-)
---
diff --git a/js/ui/background.js b/js/ui/background.js
index 20f5e89126..2811da3fb0 100644
--- a/js/ui/background.js
+++ b/js/ui/background.js
@@ -765,10 +765,19 @@ var BackgroundManager = class BackgroundManager {
             this._updateBackgroundActor();
         });
 
+        let loadedSignalId = background.connect('loaded', () => {
+            background.disconnect(loadedSignalId);
+            loadedSignalId = null;
+            this.emit('loaded');
+        });
+
         backgroundActor.connect('destroy', () => {
             if (changeSignalId)
                 background.disconnect(changeSignalId);
 
+            if (loadedSignalId)
+                background.disconnect(loadedSignalId);
+
             if (backgroundActor.loadedSignalId)
                 background.disconnect(backgroundActor.loadedSignalId);
         });
diff --git a/js/ui/layout.js b/js/ui/layout.js
index 55cefb2823..c38982ca89 100644
--- a/js/ui/layout.js
+++ b/js/ui/layout.js
@@ -457,7 +457,16 @@ var LayoutManager = GObject.registerClass({
         }
     }
 
-    _updateBackgrounds() {
+    async _waitLoaded(bgManager) {
+        return new Promise(resolve => {
+            const id = bgManager.connect('loaded', () => {
+                bgManager.disconnect(id);
+                resolve();
+            });
+        });
+    }
+
+    async _updateBackgrounds() {
         for (let i = 0; i < this._bgManagers.length; i++)
             this._bgManagers[i].destroy();
 
@@ -466,6 +475,9 @@ var LayoutManager = GObject.registerClass({
         if (Main.sessionMode.isGreeter)
             return;
 
+        let nLoadedBackgrounds = 0;
+        const nBackgrounds = this.monitors.length;
+
         for (let i = 0; i < this.monitors.length; i++) {
             let bgManager = this._createBackgroundManager(i);
             this._bgManagers.push(bgManager);
@@ -473,6 +485,8 @@ var LayoutManager = GObject.registerClass({
             if (i != this.primaryIndex && this._startingUp)
                 bgManager.backgroundActor.hide();
         }
+
+        return Promise.all(this._bgManagers.map(this._waitLoaded));
     }
 
     _updateKeyboardBox() {
@@ -631,7 +645,7 @@ var LayoutManager = GObject.registerClass({
     // When starting a normal user session, we want to grow it out of the middle
     // of the screen.
 
-    _prepareStartupAnimation() {
+    async _prepareStartupAnimation() {
         // During the initial transition, add a simple actor to block all events,
         // so they don't get delivered to X11 windows that have been transformed.
         this._coverPane = new Clutter.Actor({ opacity: 0,
@@ -648,8 +662,6 @@ var LayoutManager = GObject.registerClass({
         } else if (Main.sessionMode.isGreeter) {
             this.panelBox.translation_y = -this.panelBox.height;
         } else {
-            this._updateBackgrounds();
-
             // We need to force an update of the regions now before we scale
             // the UI group to get the correct allocation for the struts.
             this._updateRegions();
@@ -665,6 +677,8 @@ var LayoutManager = GObject.registerClass({
             this.uiGroup.scale_x = this.uiGroup.scale_y = 0.75;
             this.uiGroup.opacity = 0;
             global.window_group.set_clip(monitor.x, monitor.y, monitor.width, monitor.height);
+
+            await this._updateBackgrounds();
         }
 
         this.emit('startup-prepared');