Shrink to a floating button instead of hiding

With "Shrink to Button" on, the dismiss key shrinks the window to just
itself: a see-through button that can be dragged anywhere. Tapping it
grows the keyboard back around it, kept on screen. KWin moves and
resizes the window through a one-off script, as Wayland clients can't
place themselves.

Also moves the window back on screen whenever it appears (KWin remembers
where the button was), and releases the mouse after a window manager
move or resize, which used to eat the next press on the grab bar.

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

src/window/CollapseController.cpp

@@ -0,0 +1,74 @@
+#include "CollapseController.h"
+
+#include "FloatingWindow.h"
+
+#include <QQuickWindow>
+
+namespace {
+constexpr int ExpandTimeoutMs = 500;
+
+bool roughlyEqual(QSize a, QSize b)
+{
+    return qAbs(a.width() - b.width()) <= 1 && qAbs(a.height() - b.height()) <= 1;
+}
+} // namespace
+
+CollapseController::CollapseController(QQuickWindow &window, FloatingWindow &placement, QObject *parent)
+    : QObject(parent)
+    , m_window(window)
+    , m_placement(placement)
+{
+    m_expanding.setSingleShot(true);
+    m_expanding.setInterval(ExpandTimeoutMs);
+    connect(&m_expanding, &QTimer::timeout, this, &CollapseController::finishExpanding);
+    connect(&window, &QWindow::widthChanged, this, &CollapseController::windowResized);
+    connect(&window, &QWindow::heightChanged, this, &CollapseController::windowResized);
+}
+
+bool CollapseController::collapse()
+{
+    if (m_collapsed)
+        return true;
+    QVariant rect;
+    QMetaObject::invokeMethod(&m_window, "dismissKeyRect", Q_RETURN_ARG(QVariant, rect));
+    if (rect.toRectF().isEmpty())
+        return false;
+    m_keyRect = rect.toRectF();
+    m_expandedSize = m_window.size();
+    m_collapsed = true;
+    Q_EMIT collapsedChanged();
+    // After the next frame, once the compositor knows the lower minimum size.
+    connect(&m_window, &QQuickWindow::frameSwapped, this, [this] {
+        if (m_collapsed)
+            m_placement.moveResize(m_keyRect, false);
+    }, Qt::SingleShotConnection);
+    return true;
+}
+
+void CollapseController::expand()
+{
+    if (!m_collapsed || m_expanding.isActive())
+        return;
+    if (roughlyEqual(m_window.size(), m_expandedSize)) {
+        finishExpanding(); // never shrank: nothing to move
+        return;
+    }
+    // Stays collapsed until the window has grown, so the keys don't flash squeezed.
+    m_placement.moveResize(QRectF(-m_keyRect.topLeft(), m_expandedSize), true);
+    m_expanding.start();
+}
+
+void CollapseController::windowResized()
+{
+    if (m_expanding.isActive() && roughlyEqual(m_window.size(), m_expandedSize))
+        finishExpanding();
+}
+
+void CollapseController::finishExpanding()
+{
+    m_expanding.stop();
+    if (!roughlyEqual(m_window.size(), m_expandedSize))
+        m_window.resize(m_expandedSize); // no KWin to do it
+    m_collapsed = false;
+    Q_EMIT collapsedChanged();
+}