diff --git a/docs/backends.rst b/docs/backends.rst
index 200844a..ebdf4c7 100644
--- a/docs/backends.rst
+++ b/docs/backends.rst
@@ -40,16 +40,17 @@ but you can replace ``from rendercanvas.auto`` with ``from rendercanvas.glfw`` t
 Support for Qt
 --------------
 
-RenderCanvas has support for PyQt5, PyQt6, PySide2 and PySide6. It detects what
-qt library you are using by looking what module has been imported.
+RenderCanvas has support for PyQt5, PyQt6, PySide2 and PySide6.
 For a toplevel widget, the ``rendercanvas.qt.RenderCanvas`` class can be imported. If you want to
 embed the canvas as a subwidget, use ``rendercanvas.qt.QRenderWidget`` instead.
 
+Importing ``rendercanvas.qt`` detects what qt library is currently imported:
+
 .. code-block:: py
 
-    # Import any of the Qt libraries before importing the RenderCanvas.
-    # This way rendercanvas knows which Qt library to use.
+    # Import Qt first, otherwise rendercanvas does not know what qt-lib to use
     from PySide6 import QtWidgets
+
     from rendercanvas.qt import RenderCanvas  # use this for top-level windows
     from rendercanvas.qt import QRenderWidget  # use this for widgets in you application
 
@@ -64,6 +65,21 @@ embed the canvas as a subwidget, use ``rendercanvas.qt.QRenderWidget`` instead.
     app.exec_()
 
 
+Alternatively, you can select the specific qt library to use, making it easy to e.g. test an example on a specific Qt library.
+
+.. code-block:: py
+
+    from rendercanvas.pyside6 import RenderCanvas, loop
+
+    # Instantiate the canvas
+    canvas = RenderCanvas(title="Example")
+
+    # Tell the canvas what drawing function to call
+    canvas.request_draw(your_draw_function)
+
+    loop.run()  # calls app.exec_()
+
+
 Support for wx
 --------------
 
diff --git a/rendercanvas/_coreutils.py b/rendercanvas/_coreutils.py
index a93c36c..2519760 100644
--- a/rendercanvas/_coreutils.py
+++ b/rendercanvas/_coreutils.py
@@ -156,6 +156,18 @@ def __init__(self):
 QT_MODULE_NAMES = ["PySide6", "PyQt6", "PySide2", "PyQt5"]
 
 
+def select_qt_lib():
+    """Select the qt lib to use, used by qt.py"""
+    # Check the override. This env var is meant for internal use only.
+    # Otherwise check imported libs.
+
+    libname = os.getenv("_RENDERCANVAS_QT_LIB")
+    if libname:
+        return libname, qt_lib_has_app(libname)
+    else:
+        return get_imported_qt_lib()
+
+
 def get_imported_qt_lib():
     """Get the name of the currently imported qt lib.
 
@@ -170,13 +182,9 @@ def get_imported_qt_lib():
             imported_libs.append(libname)
 
     # Get which of these have an application object
-    imported_libs_with_app = []
-    for libname in imported_libs:
-        QtWidgets = sys.modules.get(libname + ".QtWidgets", None)  # noqa: N806
-        if QtWidgets:
-            app = QtWidgets.QApplication.instance()
-            if app is not None:
-                imported_libs_with_app.append(libname)
+    imported_libs_with_app = [
+        libname for libname in imported_libs if qt_lib_has_app(libname)
+    ]
 
     # Return findings
     if imported_libs_with_app:
@@ -187,6 +195,13 @@ def get_imported_qt_lib():
         return None, False
 
 
+def qt_lib_has_app(libname):
+    QtWidgets = sys.modules.get(libname + ".QtWidgets", None)  # noqa: N806
+    if QtWidgets:
+        app = QtWidgets.QApplication.instance()
+        return app is not None
+
+
 def asyncio_is_running():
     """Get whether there is currently a running asyncio loop."""
     asyncio = sys.modules.get("asyncio", None)
diff --git a/rendercanvas/pyqt5.py b/rendercanvas/pyqt5.py
new file mode 100644
index 0000000..3144556
--- /dev/null
+++ b/rendercanvas/pyqt5.py
@@ -0,0 +1,11 @@
+# ruff: noqa: E402, F403
+
+import os
+
+ref_libname = "PyQt5"
+os.environ["_RENDERCANVAS_QT_LIB"] = ref_libname
+
+from .qt import check_qt_libname
+from .qt import *
+
+check_qt_libname(ref_libname)
diff --git a/rendercanvas/pyqt6.py b/rendercanvas/pyqt6.py
new file mode 100644
index 0000000..da2062b
--- /dev/null
+++ b/rendercanvas/pyqt6.py
@@ -0,0 +1,11 @@
+# ruff: noqa: E402, F403
+
+import os
+
+ref_libname = "PyQt6"
+os.environ["_RENDERCANVAS_QT_LIB"] = ref_libname
+
+from .qt import check_qt_libname
+from .qt import *
+
+check_qt_libname(ref_libname)
diff --git a/rendercanvas/pyside2.py b/rendercanvas/pyside2.py
new file mode 100644
index 0000000..1cc7c5d
--- /dev/null
+++ b/rendercanvas/pyside2.py
@@ -0,0 +1,11 @@
+# ruff: noqa: E402, F403
+
+import os
+
+ref_libname = "PySide2"
+os.environ["_RENDERCANVAS_QT_LIB"] = ref_libname
+
+from .qt import check_qt_libname
+from .qt import *
+
+check_qt_libname(ref_libname)
diff --git a/rendercanvas/pyside6.py b/rendercanvas/pyside6.py
new file mode 100644
index 0000000..47549ee
--- /dev/null
+++ b/rendercanvas/pyside6.py
@@ -0,0 +1,11 @@
+# ruff: noqa: E402, F403
+
+import os
+
+ref_libname = "PySide6"
+os.environ["_RENDERCANVAS_QT_LIB"] = ref_libname
+
+from .qt import check_qt_libname
+from .qt import *
+
+check_qt_libname(ref_libname)
diff --git a/rendercanvas/qt.py b/rendercanvas/qt.py
index 5c3ddf3..4190440 100644
--- a/rendercanvas/qt.py
+++ b/rendercanvas/qt.py
@@ -20,12 +20,12 @@
     SYSTEM_IS_WAYLAND,
     get_alt_x11_display,
     get_alt_wayland_display,
-    get_imported_qt_lib,
+    select_qt_lib,
 )
 
 
 # Select GUI toolkit
-libname, already_had_app_on_import = get_imported_qt_lib()
+libname, already_had_app_on_import = select_qt_lib()
 if libname:
     QtCore = importlib.import_module(".QtCore", libname)
     QtGui = importlib.import_module(".QtGui", libname)
@@ -52,6 +52,14 @@
     )
 
 
+def check_qt_libname(expected_libname):
+    """Little helper for the qt backends that represent a specific qt lib."""
+    if expected_libname != libname:
+        raise RuntimeError(
+            f"Failed to load rendercanvas.qt with {expected_libname}, because rendercanvas.qt is already loaded with {libname}."
+        )
+
+
 # Get version
 if libname.startswith("PySide"):
     qt_version_info = QtCore.__version_info__
@@ -320,8 +328,7 @@ def _rc_get_pixel_ratio(self):
         return self.devicePixelRatioF()
 
     def _rc_set_logical_size(self, width, height):
-        if width < 0 or height < 0:
-            raise ValueError("Window width and height must not be negative")
+        width, height = int(width), int(height)
         parent = self.parent()
         if isinstance(parent, QRenderCanvas):
             parent.resize(width, height)
@@ -539,7 +546,11 @@ def init_qt(self):
     @property
     def _app(self):
         """Return global instance of Qt app instance or create one if not created yet."""
-        return QtWidgets.QApplication.instance() or QtWidgets.QApplication([])
+        # Note: PyQt6 needs the app to be stored, or it will be gc'd.
+        app = QtWidgets.QApplication.instance()
+        if app is None:
+            self._the_app = app = QtWidgets.QApplication([])
+        return app
 
     def _rc_call_soon(self, callback, *args):
         func = callback