[gnome-shell] appDisplay: Add threshold after overshoot page switches

[Florian Müllner <fmuellner src gnome org>]

commit b0c819249677c4f981f240e3cfff40a52c40787c
Author: Florian Müllner <fmuellner gnome org>
Date:   Sat Nov 23 15:06:14 2019 +0100

    appDisplay: Add threshold after overshoot page switches
    
    We currently always switch app pages when a dragged app icon
    moves outside the grid boundaries, regardless of any previous
    page switches. This makes it too easy to switch multiple pages
    accidentally, so add a small threshold that the icon has to
    move back towards the grid before allowing another page switch.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/issues/1693

 js/ui/appDisplay.js | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)
---
diff --git a/js/ui/appDisplay.js b/js/ui/appDisplay.js
index 2b60bc9a57..155f33ed47 100644
--- a/js/ui/appDisplay.js
+++ b/js/ui/appDisplay.js
@@ -40,6 +40,8 @@ var PAGE_SWITCH_TIME = 300;
 var APP_ICON_SCALE_IN_TIME = 500;
 var APP_ICON_SCALE_IN_DELAY = 700;
 
+const OVERSHOOT_THRESHOLD = 20;
+
 const SWITCHEROO_BUS_NAME = 'net.hadess.SwitcherooControl';
 const SWITCHEROO_OBJECT_PATH = '/net/hadess/SwitcherooControl';
 
@@ -376,6 +378,8 @@ var AllView = GObject.registerClass({
         this._availWidth = 0;
         this._availHeight = 0;
 
+        this._lastOvershootY = -1;
+
         Main.overview.connect('hidden', () => this.goToPage(0));
         this._grid.connect('space-opened', () => {
             let fadeEffect = this._scrollView.get_effect('fade');
@@ -775,10 +779,23 @@ var AllView = GObject.registerClass({
         let [, gridHeight] = this.get_transformed_size();
         let gridBottom = gridY + gridHeight;
 
-        // Within the grid boundaries, or already animating
-        if (dragEvent.y > gridY && dragEvent.y < gridBottom ||
-            this._adjustment.get_transition('value') != null)
+        // Already animating
+        if (this._adjustment.get_transition('value') !== null)
+            return;
+
+        // Within the grid boundaries
+        if (dragEvent.y > gridY && dragEvent.y < gridBottom) {
+            // Check whether we moved out the area of the last switch
+            if (Math.abs(this._lastOvershootY - dragEvent.y) > OVERSHOOT_THRESHOLD)
+                this._lastOvershootY = -1;
             return;
+        }
+
+        // Still in the area of the previous page switch
+        if (this._lastOvershootY >= 0)
+            return;
+
+        this._lastOvershootY = dragEvent.y;
 
         // Moving above the grid
         let currentY = this._adjustment.value;
@@ -824,6 +841,7 @@ var AllView = GObject.registerClass({
         }
 
         this._eventBlocker.visible = this._currentPopup !== null;
+        this._lastOvershootY = -1;
     }
 
     _canAccept(source) {