See-through keyboard background with optional blur

- Opacity slider for the panel behind the keys (10–100%); keys stay opaque
- Blur Behind: asks the compositor (KWin) to blur under the rounded panel
  via KF6 WindowSystem; skipped at full opacity
- Settings panel draws over the window background so changes show live,
  and scrolls when it doesn't fit

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

qml/Main.qml

@@ -18,6 +18,10 @@ Window {
     readonly property rect inputRegion: keyboard.alternates.open
         ? Qt.rect(0, 0, width, height)
         : Qt.rect(panel.x, panel.y, panel.width, panel.height)
+    // Empty when nothing shows through, so the compositor skips the blur.
+    readonly property rect blurRegion: settings.blurBackground && settings.backgroundOpacity < 1
+        ? Qt.rect(panel.x, panel.y, panel.width, panel.height)
+        : Qt.rect(0, 0, 0, 0)
 
     width: 1000
     height: 340 + popupMargin
@@ -27,13 +31,17 @@ Window {
     color: "transparent"
 
     onInputRegionChanged: placement?.setInputRegion(inputRegion)
-    onPlacementChanged: placement?.setInputRegion(inputRegion)
+    onBlurRegionChanged: placement?.setBlurRegion(blurRegion, Theme.windowRadius)
+    onPlacementChanged: {
+        placement?.setInputRegion(inputRegion);
+        placement?.setBlurRegion(blurRegion, Theme.windowRadius);
+    }
 
     Rectangle {
         id: panel
         anchors { fill: parent; topMargin: root.popupMargin }
         radius: Theme.windowRadius
-        color: Theme.background
+        color: Qt.rgba(Theme.background.r, Theme.background.g, Theme.background.b, root.settings.backgroundOpacity)
 
         Keyboard {
             id: keyboard

qml/SettingsPanel.qml

@@ -1,21 +1,25 @@
 import QtQuick
 import Sloposk
 
-// Replaces the keys while open. Binds straight to Settings, which persists.
-Rectangle {
+// Replaces the keys while open, over the window's own background so
+// opacity changes show live. Binds straight to Settings, which persists.
+Flickable {
     id: panel
 
     required property Settings settings
     required property LayoutModel layoutModel
 
-    color: Theme.background
-    radius: Theme.windowRadius
+    clip: true
+    contentHeight: columns.height + 2 * columns.y
+    boundsBehavior: Flickable.StopAtBounds
 
     Row {
         id: columns
         readonly property real columnWidth: (width - spacing) / 2
 
-        anchors { fill: parent; margins: 16 }
+        x: 16
+        y: 16
+        width: panel.width - 2 * x
         spacing: 32
 
         Column {
@@ -55,6 +59,24 @@ Rectangle {
                 checked: panel.settings.doubleSpacePeriod
                 onToggled: checked => panel.settings.doubleSpacePeriod = checked
             }
+
+            SectionHeader { text: qsTr("Appearance"); topPadding: 12 }
+
+            SliderRow {
+                width: parent.width
+                text: qsTr("Background")
+                from: 0.1 // Settings::MinBackgroundOpacity
+                value: panel.settings.backgroundOpacity
+                onMoved: value => panel.settings.backgroundOpacity = value
+            }
+
+            ToggleRow {
+                width: parent.width
+                text: qsTr("Blur Behind")
+                detail: qsTr("Frosted glass under a see-through background, if the desktop supports it")
+                checked: panel.settings.blurBackground
+                onToggled: checked => panel.settings.blurBackground = checked
+            }
         }
 
         Column {

qml/SliderRow.qml

@@ -0,0 +1,81 @@
+import QtQuick
+
+// A labelled slider in the iOS style; value runs from `from` to 1.
+Item {
+    id: row
+
+    required property string text
+    property real from: 0
+    property real value: 1
+    signal moved(real value)
+
+    implicitHeight: 52
+
+    function moveTo(x) {
+        const fraction = Math.max(0, Math.min(1, (x - knob.width / 2) / (track.width - knob.width)));
+        row.moved(row.from + fraction * (1 - row.from));
+    }
+
+    Text {
+        id: label
+        anchors { left: parent.left; verticalCenter: parent.verticalCenter }
+        width: Math.min(implicitWidth, parent.width / 3)
+        text: row.text
+        color: Theme.text
+        font.pixelSize: 16
+        elide: Text.ElideRight
+    }
+
+    Text {
+        id: percent
+        anchors { right: parent.right; verticalCenter: parent.verticalCenter }
+        width: 44
+        horizontalAlignment: Text.AlignRight
+        text: Math.round(row.value * 100) + "%"
+        color: Theme.secondaryText
+        font.pixelSize: 14
+    }
+
+    Item {
+        id: track
+        anchors { left: label.right; leftMargin: 12; right: percent.left; rightMargin: 8 }
+        height: parent.height
+
+        readonly property real fraction: (row.value - row.from) / (1 - row.from)
+
+        Rectangle {
+            anchors { left: parent.left; right: parent.right; verticalCenter: parent.verticalCenter }
+            height: 4
+            radius: 2
+            color: Theme.functionKey
+
+            Rectangle {
+                width: knob.x + knob.width / 2
+                height: parent.height
+                radius: parent.radius
+                color: Theme.accent
+            }
+        }
+
+        Rectangle {
+            id: knob
+            x: track.fraction * (track.width - width)
+            anchors.verticalCenter: parent.verticalCenter
+            width: 26
+            height: width
+            radius: width / 2
+            color: "#ffffff"
+            border { color: Theme.keyShadow; width: 0.5 }
+        }
+
+        TapHandler {
+            onTapped: eventPoint => row.moveTo(eventPoint.position.x)
+        }
+        DragHandler {
+            target: null
+            xAxis.enabled: true
+            yAxis.enabled: false
+            onCentroidChanged: if (active) row.moveTo(centroid.position.x)
+        }
+    }
+}