From 6a43e04db3e39e0b6473a65333ae3a6d9bc14df3 Mon Sep 17 00:00:00 2001
From: Igor Zinken <730069+igorski@users.noreply.github.com>
Date: Sun, 1 Mar 2026 18:19:54 +0100
Subject: [PATCH] Improved scrollbar control

---
 .../document-canvas/scrollbars/scrollbars.vue | 123 +++++++++++++-----
 1 file changed, 92 insertions(+), 31 deletions(-)

diff --git a/src/components/document-canvas/scrollbars/scrollbars.vue b/src/components/document-canvas/scrollbars/scrollbars.vue
index 935d369..01f6628 100644
--- a/src/components/document-canvas/scrollbars/scrollbars.vue
+++ b/src/components/document-canvas/scrollbars/scrollbars.vue
@@ -1,7 +1,7 @@
 /**
  * The MIT License (MIT)
  *
- * Igor Zinken 2020-2025 - https://www.igorski.nl
+ * Igor Zinken 2020-2026 - https://www.igorski.nl
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy of
  * this software and associated documentation files (the "Software"), to deal in
@@ -23,25 +23,25 @@
 <template>
     <div>
         <div
+            v-if="canScrollHorizontally"
             ref="horScroll"
             class="scroll scroll--horizontal"
             @mousedown="handlePointerDown"
             @touchstart="handlePointerDown"
         >
             <div
-                v-if="canScrollHorizontally"
                 class="scroll__handle"
                 :style="horHandleStyle"
             ></div>
         </div>
         <div
+            v-if="canScrollVertically"
             ref="verScroll"
             class="scroll scroll--vertical"
             @mousedown="handlePointerDown"
             @touchstart="handlePointerDown"
         >
             <div
-                v-if="canScrollVertically"
                 class="scroll__handle"
                 :style="verHandleStyle"
             ></div>
@@ -51,10 +51,38 @@
 
 <script lang="ts">
 import { mapState } from "vuex";
+import { type Coordinate } from "zcanvas";
 
 const MOVE_EVENTS = [ "mousemove", "touchmove", "wheel" ];
 const UP_EVENTS   = [ "mouseup", "touchend", "touchcancel" ];
 
+/**
+ * Wrapper to get uniform coordinates from a pointer / touch event
+ * regardless whether it was initiated by mouse pointer or on a touchscreen/pad.
+ * By use of preventDefault a side effect can be introduced to stop the default
+ * behaviour on touch events (as in the context of scrollbar panning it could
+ * trigger a page bounce effect)
+ */
+function getCoordinate( e: PointerEvent | TouchEvent, preventDefault = true ): Coordinate {
+    let x = e.clientX;
+    let y = e.clientY;
+
+    // touch event handling
+    const touches = e.touches || e.changedTouches;
+    if ( touches && touches.length ) {
+        x = touches[ 0 ].pageX;
+        y = touches[ 0 ].pageY;
+        if ( preventDefault ) {
+            e.preventDefault();
+        }
+    }
+    return { x, y };
+}
+
+function clamp( value: number ) {
+    return Math.min( 1, Math.max( 0, value ));
+}
+
 export default {
     emits: [ "input" ],
     props: {
@@ -86,8 +114,8 @@ export default {
         horHandleSize: 0,
         verHandlePos: 0,
         verHandleSize: 0,
-        x: 0, // the "scrollLeft" position
-        y: 0, // the "scrollTop" position
+        x: 0.5, // the "scrollLeft" position, normalized 0 - 1 range
+        y: 0.5, // the "scrollTop" position, normalized 0 - 1 range
     }),
     computed: {
         ...mapState([
@@ -110,12 +138,26 @@ export default {
         },
         canScrollVertically(): boolean {
             return this.scrollHeight > 0;
-        }
+        },
     },
     watch: {
         windowSize(): void {
             this.calcDimensions();
         },
+        canScrollHorizontally( value: boolean ): void {
+            if ( value ) {
+                this.$nextTick(() => {
+                    this.calcDimensions();
+                });
+            }
+        },
+        canScrollVertically( value: boolean ): void {
+            if ( value ) {
+                this.$nextTick(() => {
+                    this.calcDimensions();
+                });
+            }
+        },
     },
     mounted(): void {
         this.upHandler   = this.handlePointerUp.bind( this );
@@ -128,19 +170,29 @@ export default {
     },
     methods: {
         calcDimensions(): void {
-            this.trackWidth  = this.$refs.horScroll.offsetWidth;
-            this.trackHeight = this.$refs.verScroll.offsetHeight;
+            this.trackWidth  = this.$refs.horScroll?.offsetWidth  ?? this.viewportWidth;
+            this.trackHeight = this.$refs.verScroll?.offsetHeight ?? this.viewportHeight;
 
             this.scrollWidth  = this.contentWidth  - this.viewportWidth;
             this.scrollHeight = this.contentHeight - this.viewportHeight;
 
             this.horHandleSize = ( this.viewportWidth  / this.contentWidth )  * this.trackWidth;
             this.verHandleSize = ( this.viewportHeight / this.contentHeight ) * this.trackHeight;
+
+            this.$nextTick(() => {
+                this.positionHorizontalHandle();
+                this.positionVerticalHandle();
+            });
         },
         handlePointerDown( e: PointerEvent ): void {
             this.hasHorizontalScroll = e.target === this.$refs.horScroll;
             this.hasVerticalScroll   = e.target === this.$refs.verScroll;
 
+            this.startOffset = getCoordinate( e );
+            this.xAtStart = Math.max( 0.05, this.x );
+            this.yAtStart = Math.max( 0.05, this.y );
+            this.hasScrolled = false;
+
             UP_EVENTS.forEach( type => {
                 window.addEventListener( type, this.upHandler );
             });
@@ -148,48 +200,57 @@ export default {
                 window.addEventListener( type, this.moveHandler, { passive: false });
             });
         },
-        handlePointerUp(): void {
-            this.hasHorizontalScroll = false;
-            this.hasVerticalScroll   = false;
-
+        handlePointerUp( e: PointerEvent | TouchEvent ): void {
             UP_EVENTS.forEach( type => {
                 window.removeEventListener( type, this.upHandler );
             });
             MOVE_EVENTS.forEach( type => {
                 window.removeEventListener( type, this.moveHandler );
             });
+
+            if ( !this.hasScrolled && !( e instanceof TouchEvent )) {
+                if ( this.hasHorizontalScroll ) {
+                    this.x = clamp( e.offsetX / this.trackWidth );
+                    this.positionHorizontalHandle();
+                }
+                if ( this.hasVerticalScroll ) {
+                    this.y = clamp( e.offsetY / this.trackHeight );
+                    this.positionVerticalHandle();
+                }
+                this.$emit( "input", { left: this.x, top: this.y });
+            }
         },
         handleScroll( e: PointerEvent | TouchEvent ): void {
-            if ( e.target !== this.$refs.horScroll && e.target !== this.$refs.verScroll ) {
-                return;
-            }
-            let pointerX = e.offsetX;
-            let pointerY = e.offsetY;
+            const { x, y } = getCoordinate( e );
 
-            // touch event handling
-            const touches = e.touches || e.changedTouches;
-            if ( touches && touches.length ) {
-                pointerX = touches[ 0 ].pageX;
-                pointerY = touches[ 0 ].pageY;
-                e.preventDefault(); // prevent page bounce
-            }
+            // the coordinate values here are in normalized 0 - 1 range
+            
+            let targetX = this.xAtStart / this.startOffset.x * x;
+            let targetY = this.yAtStart / this.startOffset.y * y;
 
             if ( this.hasHorizontalScroll ) {
-                this.x = Math.max( 0, pointerX / this.trackWidth ); // is % of total
+                this.x = clamp( targetX );
                 this.positionHorizontalHandle();
             }
             if ( this.hasVerticalScroll ) {
-                this.y = Math.max( 0, pointerY / this.trackHeight ); // is % of total
+                this.y = clamp( targetY );
                 this.positionVerticalHandle();
             }
+            this.hasScrolled = true;
             this.$emit( "input", { left: this.x, top: this.y });
         },
-        update( left: number, top: number ): void {
-            this.x = isNaN( left ) ? 0 : left;
-            this.y = isNaN( top )  ? 0 : top;
+        update( left: number, top: number, emit = false ): void {
+            this.x = isNaN( left ) ? 0 : clamp( left );
+            this.y = isNaN( top )  ? 0 : clamp( top );
 
-            this.positionHorizontalHandle();
-            this.positionVerticalHandle();
+            this.$nextTick(() => {
+                this.positionHorizontalHandle();
+                this.positionVerticalHandle();
+
+                if ( emit ) {
+                    this.$emit( "input", { left: this.x, top: this.y });
+                }
+            });
         },
         positionHorizontalHandle(): void {
             this.horHandlePos = ( this.trackWidth - this.horHandleSize ) * this.x;