MarqueeLabel: Smooth Scrolling

This merge request reworks the MarqueeLabel to add smooth scrolling, giving it a more clean appearance while also improving scrolling when filled when characters of different lengths.

Before:

![margueeLabel_before__トリミング_](/uploads/b162949db828a0e243b0ef109456bfdd/margueeLabel_before__トリミング_.webm)

After:

![margueeLabel__トリミング_](/uploads/7ec2a2b72799a56412b08d04b2f6eb00/margueeLabel__トリミング_.webm)

components/mobileshell/qml/components/MarqueeLabel.qml

@@ -1,64 +1,119 @@
 // SPDX-FileCopyrightText: 2022 Yari Polla <skilvingr@gmail.com>
+// SPDX-FileCopyrightText: 2025 Micah Stanley <stanleymicah@proton.me>
 // SPDX-License-Identifier: LGPL-2.0-or-later
 
 import QtQuick 2.15
+import QtQuick.Controls
+import QtQuick.Layouts
+import QtQuick.Effects
 
 import org.kde.kirigami 2.20 as Kirigami
 import org.kde.plasma.components 3.0 as PlasmaComponents
+import Qt5Compat.GraphicalEffects
 
 /**
  * This is a simple marquee (flowing) label based on PlasmaComponents Label.
  */
 
-PlasmaComponents.Label {
+OpacityMask {
     id: root
+    height: row.height
 
+    // label values
     required property string inputText
+    property font font
+    property var textFormat: Text.RichText
+    // properties for the marquee label scroll speed and wait duration
+    readonly property real scrollSpeed: 0.025
+    readonly property int waitDuration: 2000
+
     readonly property string filteredText: inputText.replace(/\n/g, ' ') // remove new line characters
+    readonly property bool charactersOverflowing: txtMeter.advanceWidth > root.width // true when text is overflowing
 
-    readonly property int interval: 200 // update position every 200 ms
-    readonly property int longDuration: 300
-    readonly property int waitDuration: 900
+    // update animation values and text positions whenever the label overflows or changes
+    onFilteredTextChanged: if (root.charactersOverflowing) { textAnimationLoop.restart() }
+    onCharactersOverflowingChanged: if (charactersOverflowing) { row.scrollPosition = 0 }
 
-    readonly property int charactersOverflow: Math.ceil((txtMeter.advanceWidth - root.width) / (txtMeter.advanceWidth / filteredText.length))
-    property int step: 0
+    Item {
+        id: rowContaner
+        anchors.fill: parent
+        height: row.height
+        opacity: 0 // we display with the opacity gradient below
 
-    TextMetrics {
-        id: txtMeter
-        font: root.font
-        text: filteredText
-    }
+        // use two identical labels for scrolling so we can give the illusion of infinite scrolling
+        RowLayout {
+            id: row
+            anchors.left: parent.left
+            anchors.right: parent.right
+            anchors.top: parent.top
 
-    Timer {
-        property bool paused: false
+            property real scrollPosition: 0
 
-        interval: root.interval
-        running: visible && charactersOverflow > 0
-        repeat: true
-        onTriggered: {
-            if (paused) {
-                if (step != 0) {
-                    interval = root.longDuration;
-                    step = 0;
-                } else {
-                    interval = root.interval;
-                    paused = false;
+            transform: [
+                Translate {
+                    x: row.scrollPosition
                 }
-            } else {
-                step = (step + 1) % filteredText.length;
-                if (step === charactersOverflow) {
-                    interval = root.waitDuration;
-                    paused = true;
+            ]
+
+            spacing: 32
+
+            PlasmaComponents.Label {
+                id: label
+                font: root.font
+                textFormat: root.textFormat
+                text: filteredText
+
+                Layout.alignment: Qt.AlignLeft
+
+                TextMetrics {
+                    id: txtMeter
+                    font: root.font
+                    text: filteredText
                 }
             }
-        }
 
-        onRunningChanged: {
-            if (!running) {
-                step = 0;
+            PlasmaComponents.Label {
+                // hide this label when the text is not overflowing so the user never sees both labels
+                visible: textAnimationLoop.running
+                font: root.font
+                textFormat: root.textFormat
+                text: filteredText
+
+                Layout.alignment: Qt.AlignLeft
+                Layout.fillWidth: true
             }
         }
     }
 
-    text: filteredText.substring(step, step + filteredText.length - charactersOverflow)
+    // setting the gradient mask source
+    source: rowContaner
+
+    // if the label is overflowing, this animation in a loop smoothly scrolling thought the text
+    SequentialAnimation {
+        id: textAnimationLoop
+        running: root.charactersOverflowing && root.visible
+        onRunningChanged: row.scrollPosition = 0
+        loops: Animation.Infinite
+        PauseAnimation { duration: root.waitDuration }
+        NumberAnimation { target: row; property: "scrollPosition"; from: 0; to: -txtMeter.advanceWidth - row.spacing; duration: (txtMeter.advanceWidth + row.spacing) / root.scrollSpeed }
+    }
+
+    // gradient mask to smoothly fade the ends of the label when it is scrolling
+    maskSource: Rectangle {
+        id: mask
+        width: root.width
+        height: root.height
+
+        property real gradientPct: (Kirigami.Units.gridUnit * 0.35) / root.width
+
+        gradient: Gradient {
+            orientation: Gradient.Horizontal
+
+            GradientStop { position: 0; color: row.scrollPosition == 0 || row.scrollPosition < -txtMeter.advanceWidth ? 'white' : 'transparent' } // remove the beginning of the gradient when at the start of the label so the front text is fully visible
+            GradientStop { position: 0 + mask.gradientPct; color: 'white' }
+            GradientStop { position: 1.0 - mask.gradientPct; color: 'white' }
+            GradientStop { position: 1.0; color: 'transparent' }
+        }
+    }
 }
+