Ensure size is valid after maximising

In X11 when a window is maximised if the client is unable to fufill the
space provided we centre align the window.

With the new floating point geometry behaviour of centreing changes.
Instead of a 1 pixel gap at the top, we get a 0.5 pixel gap either side.
When we get into the codepath to "fix" the window in `closeHeight` we
only move the top, giving us an invalid buffer size.

We don't really want to change the logic here; on xwayland with the
scaling opt-out path it's feasible for a floating sized logical size to
still be representable. This code rounds to the native unit after all
the logic has taken effect.

autotests/xcb_scaling_mock.cpp

@@ -27,4 +27,8 @@ QSizeF Xcb::fromXNative(const QSize &value)
     return value;
 }
 
+QRectF Xcb::nativeFloor(const QRectF &value)
+{
+    return value;
+}
 }

src/utils/xcbutils.cpp

@@ -646,5 +646,11 @@ qreal nativeFloor(qreal value)
     return std::floor(value / kwinApp()->xwaylandScale()) * kwinApp()->xwaylandScale();
 }
 
+QRectF nativeFloor(const QRectF &value)
+{
+    return QRectF(nativeFloor(value.left()), nativeFloor(value.top()),
+                  nativeFloor(value.width()), nativeFloor(value.height()));
+}
+
 } // namespace Xcb
 } // namespace KWin

src/utils/xcbutils.h

@@ -47,6 +47,7 @@ QSizeF KWIN_EXPORT fromXNative(const QSize &value);
  *  i.e floor(a/scale) * scale
  */
 qreal KWIN_EXPORT nativeFloor(qreal value);
+QRectF KWIN_EXPORT nativeFloor(const QRectF &value);
 
 // forward declaration of methods
 static void defineCursor(xcb_window_t window, xcb_cursor_t cursor);

src/x11window.cpp

@@ -4536,6 +4536,10 @@ void X11Window::changeMaximize(bool horizontal, bool vertical, bool adjust)
             }
             r.moveTopLeft(rules()->checkPosition(r.topLeft()));
         }
+        // The above code tries to center align the window followed by setting top and bottom
+        // it's possible that we no longer have a valid size
+        r = Xcb::nativeFloor(r);
+
         moveResize(r);
         if (options->electricBorderMaximize() && r.top() == clientArea.top()) {
             updateQuickTileMode(QuickTileFlag::Maximize);