Fake input: check the bound protocol version

Qt binds the lower of the asked and advertised versions; sending
keyboard_keysym (v6) or destroy (v5) to an older KWin makes libwayland
abort. Those requests are skipped there, with a warning.

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

src/backend/KWinFakeInput.cpp

@@ -1,9 +1,13 @@
 #include "KWinFakeInput.h"
 
+#include <QtLogging>
 #include <xkbcommon/xkbcommon.h>
 
 namespace {
 
+constexpr uint32_t KeysymVersion = 6; // keyboard_keysym; keyboard_key is 4
+constexpr uint32_t DestroyVersion = 5;
+
 constexpr uint32_t Released = 0;
 constexpr uint32_t Pressed = 1;
 
@@ -22,10 +26,14 @@ constexpr ModifierKey ModifierKeys[] = {
 } // namespace
 
 KWinFakeInput::KWinFakeInput()
-    : QWaylandClientExtensionTemplate(6) // 6: keyboard_keysym
+    : QWaylandClientExtensionTemplate(KeysymVersion)
 {
     connect(this, &QWaylandClientExtension::activeChanged, this, [this] {
-        if (isActive())
+        if (!isActive())
+            return;
+        if (!canSendKeysyms())
+            qWarning("KWin's fake input is too old (version %u, need %u): typing outside text fields is off",
+                     boundVersion(), KeysymVersion);
         authenticate(QStringLiteral("sloposk"), QStringLiteral("On-screen keyboard: type into any window"));
     });
     initialize();
@@ -33,13 +41,23 @@ KWinFakeInput::KWinFakeInput()
 
 KWinFakeInput::~KWinFakeInput()
 {
-    if (isActive())
+    if (isActive() && boundVersion() >= DestroyVersion)
         destroy();
 }
 
+uint32_t KWinFakeInput::boundVersion() const
+{
+    return QtWayland::org_kde_kwin_fake_input::version(); // the proxy's, not the one asked for
+}
+
+bool KWinFakeInput::canSendKeysyms() const
+{
+    return isActive() && boundVersion() >= KeysymVersion;
+}
+
 void KWinFakeInput::sendKeysym(quint32 keysym, Qt::KeyboardModifiers modifiers)
 {
-    if (!isActive())
+    if (!canSendKeysyms())
         return;
     for (const ModifierKey &key : ModifierKeys) {
         if (modifiers & key.flag)
@@ -55,7 +73,7 @@ void KWinFakeInput::sendKeysym(quint32 keysym, Qt::KeyboardModifiers modifiers)
 
 void KWinFakeInput::setKeyPressed(quint32 keysym, bool pressed)
 {
-    if (isActive())
+    if (canSendKeysyms())
         keyboard_keysym(keysym, pressed ? Pressed : Released);
 }
 

src/backend/KWinFakeInput.h

@@ -24,4 +24,10 @@ public:
     void setKeyPressed(quint32 keysym, bool pressed);
     // Types each character as its keysym.
     void typeText(const QString &text);
+
+private:
+    // The version KWin bound, which may be older than the one asked for:
+    // sending a request it lacks makes libwayland abort.
+    uint32_t boundVersion() const;
+    bool canSendKeysyms() const;
 };