diff --git a/guake/utils.py b/guake/utils.py
index dc07c80e6..77991c5fd 100644
--- a/guake/utils.py
+++ b/guake/utils.py
@@ -311,7 +311,10 @@ def set_final_window_rect(cls, settings, window):
             window.maximize()
         elif not FullscreenManager(settings, window).is_fullscreen():
             log.debug("RESIZING MAIN WINDOW TO THE FOLLOWING VALUES:")
-            window.unmaximize()
+            # KWin marks a full-width window as maximized horizontally. Calling
+            # unmaximize() on it makes KWin shrink it to 2/3 of the screen width (#2117).
+            if window.is_maximized():
+                window.unmaximize()
             log.debug("  window_rect.x: %s", window_rect.x)
             log.debug("  window_rect.y: %s", window_rect.y)
             log.debug("  window_rect.height: %s", window_rect.height)
diff --git a/releasenotes/notes/bugfix-3b43f5126a83b026.yaml b/releasenotes/notes/bugfix-3b43f5126a83b026.yaml
new file mode 100644
index 000000000..00ac03fe0
--- /dev/null
+++ b/releasenotes/notes/bugfix-3b43f5126a83b026.yaml
@@ -0,0 +1,5 @@
+fixes:
+  - |
+    On KDE Plasma, a window with 100% width no longer shrinks to 2/3 of the screen width
+    when it is shown or leaves fullscreen. Guake now only unmaximizes the window when it
+    is maximized. #2117