[gnome-shell/gnome-3-38] workspacesView: Don't update scroll position when allocating

[Marge Bot <marge-bot src gnome org>]

commit f60ca05191ad5c335bbe9b1417a4f158814fc308
Author: Jonas Ådahl <jadahl gmail com>
Date:   Wed Dec 9 19:32:01 2020 +0100

    workspacesView: Don't update scroll position when allocating
    
    Updating scroll position may have significant side effects, e.g.
    switching workspace; this should never happen during allocation, as
    we're in the middle of painting a frame. So, put it in an idle callback
    if we're doing it from an allocation to have the side effects happen the
    right time.
    
    (cherry picked from commit 78d6b13c2920cadfb6efa8fb42490cd0b5f65add)
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1540>

 js/ui/workspacesView.js | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)
---
diff --git a/js/ui/workspacesView.js b/js/ui/workspacesView.js
index 4aac1cc1cd..cccd447e26 100644
--- a/js/ui/workspacesView.js
+++ b/js/ui/workspacesView.js
@@ -78,6 +78,7 @@ class WorkspacesView extends WorkspacesViewBase {
 
         this._animating = false; // tweening
         this._gestureActive = false; // touch(pad) gestures
+        this._updateScrollPositionId = 0;
 
         this._scrollAdjustment = scrollAdjustment;
         this._onScrollId = this._scrollAdjustment.connect('notify::value',
@@ -124,7 +125,14 @@ class WorkspacesView extends WorkspacesViewBase {
             child.allocate_available_size(x, y, box.get_width(), box.get_height());
         });
 
-        this._updateScrollPosition();
+        // Update scroll position in an idle callback to avoid unintended
+        // side effects (e.g. workspace switch) during layout.
+        this._updateScrollPositionId =
+            GLib.idle_add(GLib.PRIORITY_DEFAULT, () => {
+                this._updateScrollPositionId = 0;
+                this._updateScrollPosition();
+                return GLib.SOURCE_REMOVE;
+            });
     }
 
     getActiveWorkspace() {
@@ -227,6 +235,8 @@ class WorkspacesView extends WorkspacesViewBase {
     _onDestroy() {
         super._onDestroy();
 
+        if (this._updateScrollPositionId)
+            GLib.source_remove(this._updateScrollPositionId);
         this._scrollAdjustment.disconnect(this._onScrollId);
         global.window_manager.disconnect(this._switchWorkspaceNotifyId);
         let workspaceManager = global.workspace_manager;
@@ -254,6 +264,11 @@ class WorkspacesView extends WorkspacesViewBase {
         if (!this.has_allocation())
             return;
 
+        if (this._updateScrollPositionId) {
+            GLib.source_remove(this._updateScrollPositionId);
+            this._updateScrollPositionId = 0;
+        }
+
         const adj = this._scrollAdjustment;
         const allowSwitch =
             adj.get_transition('value') === null && !this._gestureActive;