[gnome-shell] workspace: Sort windows in overview grid using cached center

[Florian Müllner <fmuellner src gnome org>]

commit d91927674d4ea5ae7c7e77ace28f5939553147d5
Author: Andrew Watson <andrew l watson btinternet com>
Date:   Tue Oct 15 12:35:18 2019 +0100

    workspace: Sort windows in overview grid using cached center
    
    When accessing properties on ClutterActor for size and position there is
    a notable access time overhead. This overhead adds considerable user lag
    when opening the overview if many windows are open.
    
    This is primarily due to these properties being accessed while sorting
    WindowClone instances by their window's center for placement in the
    overview. By pre-computing this center value only once when
    initializing WindowClone, the induced lag can be significantly reduced.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/763

 js/ui/workspace.js | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)
---
diff --git a/js/ui/workspace.js b/js/ui/workspace.js
index ac53fa41cd..acad9eaf70 100644
--- a/js/ui/workspace.js
+++ b/js/ui/workspace.js
@@ -158,6 +158,8 @@ var WindowClone = GObject.registerClass({
         this.x = this._boundingBox.x;
         this.y = this._boundingBox.y;
 
+        this._computeWindowCenter();
+
         let clickAction = new Clutter.ClickAction();
         clickAction.connect('clicked', this._onClicked.bind(this));
         clickAction.connect('long-press', this._onLongPress.bind(this));
@@ -293,6 +295,18 @@ var WindowClone = GObject.registerClass({
         this.layout_manager.boundingBox = rect;
     }
 
+    get windowCenter() {
+        return this._windowCenter;
+    }
+
+    _computeWindowCenter() {
+        let box = this.realWindow.get_allocation_box();
+        this._windowCenter = new Clutter.Point({
+            x: box.get_x() + box.get_width() / 2,
+            y: box.get_y() + box.get_height() / 2,
+        });
+    }
+
     // Find the actor just below us, respecting reparenting done by DND code
     getActualStackAbove() {
         if (this._stackAbove == null)
@@ -1017,11 +1031,7 @@ var UnalignedLayoutStrategy = class extends LayoutStrategy {
     _sortRow(row) {
         // Sort windows horizontally to minimize travel distance.
         // This affects in what order the windows end up in a row.
-        row.windows.sort((a, b) => {
-            let aCenter = a.realWindow.x + a.realWindow.width / 2;
-            let bCenter = b.realWindow.x + b.realWindow.width / 2;
-            return aCenter - bCenter;
-        });
+        row.windows.sort((a, b) => a.windowCenter.x - b.windowCenter.x);
     }
 
     computeLayout(windows, layout) {
@@ -1040,11 +1050,7 @@ var UnalignedLayoutStrategy = class extends LayoutStrategy {
         // Sort windows vertically to minimize travel distance.
         // This affects what rows the windows get placed in.
         let sortedWindows = windows.slice();
-        sortedWindows.sort((a, b) => {
-            let aCenter = a.realWindow.y + a.realWindow.height / 2;
-            let bCenter = b.realWindow.y + b.realWindow.height / 2;
-            return aCenter - bCenter;
-        });
+        sortedWindows.sort((a, b) => a.windowCenter.y - b.windowCenter.y);
 
         let windowIdx = 0;
         for (let i = 0; i < numRows; i++) {