Also weigh preference of context in present-method selection

rendercanvas/base.py

@@ -670,8 +670,9 @@ def _rc_get_present_info(self, present_methods: list[str]) -> dict | None:
         selected presentation method.
 
         The ``present_methods`` represents the supported methods of the
-        canvas-context, possibly filtered by a user-specified method. A canvas
-        backend must implement at least the "screen" or "bitmap" method.
+        canvas-context, in order of context-preference, possibly filtered by a
+        user-specified method. A canvas backend must implement at least the
+        "screen" or "bitmap" method.
 
         The returned dict must contain at least the key 'method', which must
         match one of the ``present_methods``. The remaining values represent

rendercanvas/contexts/bitmapcontext.py

@@ -14,7 +14,7 @@ class BitmapContext(BaseContext):
 
     # Note:  instantiating this class creates an instance of a sub-class, dedicated to the present method of the canvas.
 
-    present_methods = ["bitmap", "screen"]
+    present_methods = ["bitmap", "screen"]  # in order of preference
 
     def __new__(cls, present_info: dict):
         # Instantiating this class actually produces a subclass

rendercanvas/contexts/wgpucontext.py

@@ -16,7 +16,7 @@ class WgpuContext(BaseContext):
 
     # Note:  instantiating this class creates an instance of a sub-class, dedicated to the present method of the canvas.
 
-    present_methods = ["screen", "bitmap"]
+    present_methods = ["screen", "bitmap"]  # in order of preference
 
     def __new__(cls, present_info: dict):
         # Instantiating this class actually produces a subclass

rendercanvas/pyodide.py

@@ -428,14 +428,18 @@ def _rc_gui_poll(self):
         pass  # Nothing to be done; the JS loop is always running (and Pyodide wraps that in a global asyncio loop)
 
     def _rc_get_present_info(self, present_methods):
-        if "screen" in present_methods:
+        # Select method
+        the_method = present_methods[0]
+
+        # Apply
+        if the_method == "screen":
             # wgpu-specific presentation. The wgpu.backends.pyodide.GPUCanvasContext must be able to consume this.
             return {
                 "method": "screen",
                 "platform": "browser",
                 "window": self._canvas_element,  # Just provide the canvas object
             }
-        elif "bitmap" in present_methods:
+        elif the_method == "bitmap":
             # Generic presentation
             return {
                 "method": "bitmap",

rendercanvas/qt.py

@@ -352,23 +352,14 @@ def _rc_gui_poll(self):
             loop._app.processEvents()
 
     def _rc_get_present_info(self, present_methods):
-        # Select what method the canvas prefers
-        preferred_method = "screen"
-        if SYSTEM_IS_WAYLAND:
+        # Select the method
+        the_method = present_methods[0]
+        if SYSTEM_IS_WAYLAND and "bitmap" in present_methods:
             # Trying to render to screen on Wayland segfaults. This might be because
             # the "display" is not the real display id. We can tell Qt to use
             # XWayland, so we can use the X11 path. This worked at some point,
             # but later this resulted in a Rust panic. So, until this is sorted
             # out, we fall back to rendering via an image.
-            preferred_method = "bitmap"
-
-        # Select method
-        the_method = None
-        if preferred_method in present_methods:
-            the_method = preferred_method
-        elif "screen" in present_methods:
-            the_method = "screen"
-        elif "bitmap" in present_methods:
             the_method = "bitmap"
 
         # Apply

rendercanvas/wx.py

@@ -282,19 +282,10 @@ def _rc_gui_poll(self):
             loop.process_wx_events()
 
     def _rc_get_present_info(self, present_methods):
-        # Select what method the canvas prefers
-        preferred_method = "screen"
-        if SYSTEM_IS_WAYLAND:
-            preferred_method = "bitmap"  # also see qt.py
-
         # Select method
-        the_method = None
-        if preferred_method in present_methods:
-            the_method = preferred_method
-        elif "screen" in present_methods:
-            the_method = "screen"
-        elif "bitmap" in present_methods:
-            the_method = "bitmap"
+        the_method = present_methods[0]
+        if SYSTEM_IS_WAYLAND and "bitmap" in present_methods:
+            the_method = "bitmap"  # also see qt.py
 
         # Apply
         if the_method == "screen":

tests/test_loop.py

@@ -395,7 +395,7 @@ def test_run_loop_and_close_canvases(SomeLoop):
     et = time.time() - t0
 
     print(et)
-    assert 0.25 < et < 0.45 + leeway
+    assert 0.25 < et < 0.50 + leeway
 
     assert canvas1._events.is_closed
     assert canvas2._events.is_closed