Add event filter for key press/release events while KWin grabbed keyboard on root window

Summary:
The modifier-only-shortcuts break as soon as KWin grabs the keyboard
(e.g. alt+tab, present windows, etc.). The investigation shows that in
that case KWin does not get any raw key events any more and thus gets
confused about the state of the hold modifiers. E.g. alt+tab has the
alt key pressed, but we miss the release as the keyboard is grabbed.

This change addresses the problem by installing an additional event
filter for key press and release event which only filters for key events
on the root window. That way we can be sure that it only operates when
KWin grabbed the keyboard on the root window.

Note: the problem only exists when grabbing on the root window. If the
grab is on another window (e.g. moving a window) we still do get all
events.

The problem also seems to not happen if another application grabbed
keys on the root window. E.g. for key combinations grabbed by
kglobalaccel the correct sequence of key press/release as raw events
are reported. Also while the screen is locked the evemts are reported
and kscreenlocker grabs the keyboard on the root window.

Test Plan:
Used Alt+Tab and Present Windows and tried to activate launcher
afterwards.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D2980

plugins/platforms/x11/standalone/xinputintegration.cpp

@@ -132,6 +132,35 @@ private:
     Xkb *m_xkb = nullptr;
 };
 
+class XKeyPressReleaseEventFilter : public X11EventFilter
+{
+public:
+    XKeyPressReleaseEventFilter(uint32_t type)
+        : X11EventFilter(type)
+    {}
+    ~XKeyPressReleaseEventFilter() = default;
+
+    bool event(xcb_generic_event_t *event) override {
+        xcb_key_press_event_t *ke = reinterpret_cast<xcb_key_press_event_t *>(event);
+        if (m_xkb && ke->event == ke->root) {
+            const uint8_t eventType = event->response_type & ~0x80;
+            if (eventType == XCB_KEY_PRESS) {
+                m_xkb->updateKey(ke->detail - 8, InputRedirection::KeyboardKeyPressed);
+            } else {
+                m_xkb->updateKey(ke->detail - 8, InputRedirection::KeyboardKeyReleased);
+            }
+        }
+        return false;
+    }
+
+    void setXkb(Xkb *xkb) {
+        m_xkb = xkb;
+    }
+
+private:
+    // TODO: QPointer
+    Xkb *m_xkb = nullptr;
+};
 
 XInputIntegration::XInputIntegration(QObject *parent)
     : QObject(parent)
@@ -207,6 +236,10 @@ void XInputIntegration::startListening()
     m_xiEventFilter.reset(new XInputEventFilter(m_xiOpcode));
     m_xiEventFilter->setCursor(m_x11Cursor);
     m_xiEventFilter->setXkb(m_xkb);
+    m_keyPressFilter.reset(new XKeyPressReleaseEventFilter(XCB_KEY_PRESS));
+    m_keyPressFilter->setXkb(m_xkb);
+    m_keyReleaseFilter.reset(new XKeyPressReleaseEventFilter(XCB_KEY_RELEASE));
+    m_keyReleaseFilter->setXkb(m_xkb);
 }
 
 }

plugins/platforms/x11/standalone/xinputintegration.h

@@ -28,6 +28,7 @@ namespace KWin
 {
 
 class XInputEventFilter;
+class XKeyPressReleaseEventFilter;
 class X11Cursor;
 class Xkb;
 
@@ -58,6 +59,8 @@ private:
     Xkb *m_xkb = nullptr;
 
     QScopedPointer<XInputEventFilter> m_xiEventFilter;
+    QScopedPointer<XKeyPressReleaseEventFilter> m_keyPressFilter;
+    QScopedPointer<XKeyPressReleaseEventFilter> m_keyReleaseFilter;
 };
 
 }