A float button in every window's title bar

Frametop's own window decoration (decoration/), a QML decoration for KWin's
Aurorae engine drawn like Breeze, puts a float button left of Close. It's the
Keep Below button with its own glyph: the KWin script floats a window when
keep-below is set and docks it when it's cleared, and keeps the flag set on
every floating window, so the button shows "back to the desktop" there. The
session script installs the decoration for the Frametop desktop only;
decoration/apply.sh switches a running desktop to it or back to Breeze.

Not yet tried in a running KWin.

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

decoration/contents/ui/FtButton.qml

@@ -0,0 +1,120 @@
+/*
+    A title bar button drawn the way Breeze draws its own: a glyph in the title's colour,
+    a soft circle behind it on hover, and a red circle for Close.
+
+    The float button is KWin's Keep Below button with a glyph of its own: Frametop's KWin
+    script (float/frametop-float.js) floats a window when keep-below is set on it and docks
+    it when it's cleared, and keeps the flag set on every floating window, so "toggled" here
+    means "floating" and the glyph turns into "back to the desktop".
+
+    SPDX-License-Identifier: GPL-2.0-or-later
+*/
+import QtQuick
+import QtQuick.Shapes
+import org.kde.kwin.decoration
+
+DecorationButton {
+    id: button
+    property real size: 20
+    property color fg: "white"
+    property color bg: "black"
+    readonly property bool isClose: buttonType === DecorationOptions.DecorationButtonClose
+    readonly property bool isFloat: buttonType === DecorationOptions.DecorationButtonKeepBelow
+    // The glyph, as polylines in an 18 x 18 box (Breeze's own sizes).
+    readonly property var glyph: {
+        switch (buttonType) {
+        case DecorationOptions.DecorationButtonClose:
+            return [[[5, 5], [13, 13]], [[13, 5], [5, 13]]];
+        case DecorationOptions.DecorationButtonMaximizeRestore:
+            return decoration.client.maximized ? [[[4.5, 9], [9, 4.5], [13.5, 9], [9, 13.5], [4.5, 9]]]
+                                               : [[[4, 11.5], [9, 6.5], [14, 11.5]]];
+        case DecorationOptions.DecorationButtonMinimize:
+            return [[[4, 7], [9, 12], [14, 7]]];
+        case DecorationOptions.DecorationButtonKeepAbove:
+            return [[[4, 9], [9, 4], [14, 9]], [[4, 14], [9, 9], [14, 14]]];
+        case DecorationOptions.DecorationButtonShade:
+            return [[[4, 5], [14, 5]], [[4, 13], [9, 8], [14, 13]]];
+        case DecorationOptions.DecorationButtonApplicationMenu:
+            return [[[3.5, 5], [14.5, 5]], [[3.5, 9], [14.5, 9]], [[3.5, 13], [14.5, 13]]];
+        case DecorationOptions.DecorationButtonKeepBelow:
+            // A box with an arrow leaving it (float in VR), or coming back into it (put it back).
+            return button.toggled
+                ? [[[8, 3.5], [3.5, 3.5], [3.5, 14.5], [14.5, 14.5], [14.5, 10]], [[14.5, 3.5], [8.5, 9.5]],
+                   [[8.5, 5], [8.5, 9.5], [13, 9.5]]]
+                : [[[8, 3.5], [3.5, 3.5], [3.5, 14.5], [14.5, 14.5], [14.5, 10]], [[8.5, 9.5], [14.5, 3.5]],
+                   [[10, 3.5], [14.5, 3.5], [14.5, 8]]];
+        }
+        return [];
+    }
+    readonly property string tip: isFloat ? (toggled ? "Back to Desktop" : "Float in VR") : ""
+
+    width: size
+    height: size
+
+    Rectangle {
+        anchors.fill: parent
+        radius: width / 2
+        visible: button.hovered || button.pressed || (button.toggled && !button.isFloat)
+        color: button.isClose ? (button.pressed ? "#c0392b" : "#da4453")
+                              : Qt.rgba(button.fg.r, button.fg.g, button.fg.b,
+                                        button.pressed ? 0.35 : button.hovered ? 0.2 : 0.12)
+    }
+
+    // On all desktops: a dot, filled while the window is on all of them. Help: a question mark.
+    Rectangle {
+        visible: button.buttonType === DecorationOptions.DecorationButtonOnAllDesktops
+        anchors.centerIn: parent
+        width: button.size * 0.3
+        height: width
+        radius: width / 2
+        color: button.toggled ? button.fg : "transparent"
+        border.width: Math.max(1, button.size / 18)
+        border.color: button.fg
+    }
+    Text {
+        visible: button.buttonType === DecorationOptions.DecorationButtonQuickHelp
+        anchors.centerIn: parent
+        text: "?"
+        color: button.fg
+        font.pixelSize: button.size * 0.65
+        font.bold: true
+    }
+
+    // Up to three strokes per glyph.
+    function stroke(i) {
+        const k = button.size / 18;
+        return i < glyph.length ? glyph[i].map(p => Qt.point(p[0] * k, p[1] * k)) : [];
+    }
+    readonly property color strokeColor: isClose && (hovered || pressed) ? "white" : fg
+    readonly property real strokeWidth: Math.max(1, size / 18 * 1.1)
+    // (An inline component can't see the ids around it, so everything comes in as properties.)
+    component Stroke: ShapePath {
+        property var points: []
+        fillColor: "transparent"
+        capStyle: ShapePath.RoundCap
+        joinStyle: ShapePath.RoundJoin
+        PathPolyline { path: points }
+    }
+    Shape {
+        anchors.fill: parent
+        preferredRendererType: Shape.CurveRenderer
+        Stroke { points: button.stroke(0); strokeColor: button.strokeColor; strokeWidth: button.strokeWidth }
+        Stroke { points: button.stroke(1); strokeColor: button.strokeColor; strokeWidth: button.strokeWidth }
+        Stroke { points: button.stroke(2); strokeColor: button.strokeColor; strokeWidth: button.strokeWidth }
+    }
+
+    onHoveredChanged: {
+        if (!tip || typeof decoration.requestShowToolTip !== "function")
+            return;
+        if (hovered)
+            decoration.requestShowToolTip(tip);
+        else
+            decoration.requestHideToolTip();
+    }
+    Component.onCompleted: {
+        if (buttonType === DecorationOptions.DecorationButtonQuickHelp)
+            visible = Qt.binding(() => decoration.client.providesContextHelp);
+        if (buttonType === DecorationOptions.DecorationButtonApplicationMenu)
+            visible = Qt.binding(() => decoration.client.hasApplicationMenu);
+    }
+}