Trigger resize of input window when deco size changes

Summary:
Client::updateInputWindow operates with the decoration size. The method
gets called from various points when changing the window geometry. If at
that moment the decoration has not updated yet, the borders might be at
a wrong position.

This behavior could be triggered when a window requested to change the
state to maximized. During maximization the decoration still had the
wrong size when updateInputWindow was called, thus an interactive area
inside the window was created.

To circumvent this problem updateInputWindow is now also called whenever
the window decoration changes.

As a note: that a maximized window has resize only borders is wrong. Kwin
should be protected against that.

BUG: 371284
FIXED-IN: 5.8.3

Test Plan: Checked xwininfo for the deco extends window

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3151

client.cpp

@@ -410,6 +410,8 @@ void Client::createDecoration(const QRect& oldgeom)
                 emit geometryShapeChanged(this, oldgeom);
             }
         );
+        connect(decoratedClient()->decoratedClient(), &KDecoration2::DecoratedClient::widthChanged, this, &Client::updateInputWindow);
+        connect(decoratedClient()->decoratedClient(), &KDecoration2::DecoratedClient::heightChanged, this, &Client::updateInputWindow);
     }
     setDecoration(decoration);