[gnome-shell] appDisplay: Reimplement drag motion using page indicators

[Robert Mader <rmader src gnome org>]

commit 09b975fabf8e05ca2c777a498364672701808dcc
Author: Georges Basile Stavracas Neto <georges stavracas gmail com>
Date:   Tue Jun 28 19:47:06 2022 -0300

    appDisplay: Reimplement drag motion using page indicators
    
    This simplified the _handleDragOvershoot() callback quite a lot.
    Instead of transforming point positions and checking them against
    grid coordinates, merely check if the drop target is one of the
    page indicators.
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2335>

 js/ui/appDisplay.js | 47 ++++++++++++++---------------------------------
 1 file changed, 14 insertions(+), 33 deletions(-)
---
diff --git a/js/ui/appDisplay.js b/js/ui/appDisplay.js
index 8689e62a26..cc7881c6e1 100644
--- a/js/ui/appDisplay.js
+++ b/js/ui/appDisplay.js
@@ -43,7 +43,6 @@ const PAGE_PREVIEW_ANIMATION_TIME = 150;
 const PAGE_INDICATOR_FADE_TIME = 200;
 const PAGE_PREVIEW_RATIO = 0.20;
 
-const OVERSHOOT_THRESHOLD = 20;
 const OVERSHOOT_TIMEOUT = 1000;
 
 const DELAYED_MOVE_TIMEOUT = 200;
@@ -548,7 +547,7 @@ var BaseAppView = GObject.registerClass({
             style_class: 'page-navigation-hint next',
             opacity: 0,
             visible: false,
-            reactive: false,
+            reactive: true,
             x_expand: true,
             y_expand: true,
             x_align: Clutter.ActorAlign.FILL,
@@ -559,7 +558,7 @@ var BaseAppView = GObject.registerClass({
             style_class: 'page-navigation-hint previous',
             opacity: 0,
             visible: false,
-            reactive: false,
+            reactive: true,
             x_expand: true,
             y_expand: true,
             x_align: Clutter.ActorAlign.FILL,
@@ -641,7 +640,6 @@ var BaseAppView = GObject.registerClass({
             () => this._redisplay(), this);
 
         // Drag n' Drop
-        this._lastOvershoot = -1;
         this._lastOvershootTimeoutId = 0;
         this._delayedMoveData = null;
 
@@ -846,54 +844,37 @@ var BaseAppView = GObject.registerClass({
         if (this._lastOvershootTimeoutId)
             GLib.source_remove(this._lastOvershootTimeoutId);
         this._lastOvershootTimeoutId = 0;
-        this._lastOvershoot = -1;
     }
 
     _handleDragOvershoot(dragEvent) {
-        const [gridX, gridY] = this.get_transformed_position();
-        const [gridWidth, gridHeight] = this.get_transformed_size();
-
-        const vertical = this._orientation === Clutter.Orientation.VERTICAL;
-        const gridStart = vertical ? gridY : gridX;
-        const gridEnd = vertical
-            ? gridY + gridHeight - OVERSHOOT_THRESHOLD
-            : gridX + gridWidth - OVERSHOOT_THRESHOLD;
-
         // Already animating
         if (this._adjustment.get_transition('value') !== null)
             return;
 
-        // Within the grid boundaries
-        const dragPosition = vertical ? dragEvent.y : dragEvent.x;
-        if (dragPosition > gridStart && dragPosition < gridEnd) {
-            // Check whether we moved out the area of the last switch
-            if (Math.abs(this._lastOvershoot - dragPosition) > OVERSHOOT_THRESHOLD)
-                this._resetOvershoot();
+        const {targetActor} = dragEvent;
 
+        if (targetActor !== this._prevPageIndicator &&
+            targetActor !== this._nextPageIndicator) {
+            this._resetOvershoot();
             return;
         }
 
-        // Still in the area of the previous page switch
-        if (this._lastOvershoot >= 0)
+        if (this._lastOvershootTimeoutId > 0)
             return;
 
-        const rtl = this.get_text_direction() === Clutter.TextDirection.RTL;
-        if (dragPosition <= gridStart)
-            this.goToPage(this._grid.currentPage + (rtl ? 1 : -1));
-        else if (dragPosition >= gridEnd)
-            this.goToPage(this._grid.currentPage + (rtl ? -1 : 1));
+        let targetPage;
+        if (dragEvent.targetActor === this._prevPageIndicator)
+            targetPage = this._grid.currentPage - 1;
         else
-            return; // don't go beyond first/last page
-
-        this._lastOvershoot = dragPosition;
+            targetPage = this._grid.currentPage + 1;
 
-        if (this._lastOvershootTimeoutId > 0)
-            GLib.source_remove(this._lastOvershootTimeoutId);
+        if (targetPage < 0 || targetPage >= this._grid.nPages)
+            return; // don't go beyond first/last page
 
         this._lastOvershootTimeoutId =
             GLib.timeout_add(GLib.PRIORITY_DEFAULT, OVERSHOOT_TIMEOUT, () => {
                 this._resetOvershoot();
-                this._handleDragOvershoot(dragEvent);
+                this.goToPage(targetPage);
                 return GLib.SOURCE_REMOVE;
             });
         GLib.Source.set_name_by_id(this._lastOvershootTimeoutId,