fix(juggler): never enter Responsive Design Mode, and warn about is_mobile

#798 left RDM on for isMobile, as Playwright's Juggler does. RDM matches
no real browser: Firefox for Android never runs it, and under touch
emulation it drops a mouse click's pointer events, which no device does.
Camoufox only has desktop identities, so an is_mobile context was a
desktop UA, platform, fonts and GPU with devtools' mobile mode on top.

Juggler now keeps inRDMPane off for every page, is_mobile included, and
the isMobile plumbing #798 added is gone again. viewport-no-rdm requires
is_mobile=True to keep the platform's scrollbars too.

Both launchers warn instead (warnings.yml is_mobile): on new_page() /
new_context(is_mobile=True) of a Camoufox browser, and on is_mobile
passed to launch_options() for a persistent context. has_touch,
device_scale_factor and viewport keep working without RDM.

Upstream playwright-python skips its isMobile tests on Firefox in
1.61-1.63, so no skiplist entries are needed.

On beta.31 with this Juggler in omni.ja, viewport-no-rdm passes: 12/12 px
of scrollbar with no viewport, with a viewport, and with is_mobile, and a
has_touch click fires pointerdown and pointerup. pythonlib 411 passed;
typescript 584 passed.

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

pythonlib/tests/test_identity_salt.py

@@ -191,3 +191,11 @@ def test_instant_animations_warn_that_they_are_detectable():
 
     with pytest.warns(LeakWarning, match="getComputedTiming"):
         launch(config={"instantAnimations": True}, i_know_what_im_doing=False)
+
+
+def test_is_mobile_warns_that_camoufox_is_desktop_only():
+    """A persistent context takes its context options at launch."""
+    from camoufox._warnings import LeakWarning
+
+    with pytest.warns(LeakWarning, match="built for desktops"):
+        launch(is_mobile=True, i_know_what_im_doing=False)

pythonlib/tests/test_launch_environment.py

@@ -388,3 +388,32 @@ class TestStockMediaDefaults:
         assert browser.kwargs["forced_colors"] == "active"
         # the ones the caller left alone still follow the host
         assert browser.kwargs["reduced_motion"] == "no-override"
+
+
+class TestDesktopOnlyWarning:
+    """Camoufox has only desktop identities and Juggler never enters Responsive
+    Design Mode, so is_mobile on new_page()/new_context() gets a LeakWarning."""
+
+    class FakeBrowser:
+        def new_page(self, **kwargs):
+            return kwargs
+
+        def new_context(self, **kwargs):
+            return kwargs
+
+    def test_is_mobile_warns(self):
+        from camoufox._warnings import LeakWarning
+
+        browser = utils.attach_desktop_only_warning(self.FakeBrowser())
+        for create in (browser.new_page, browser.new_context):
+            with pytest.warns(LeakWarning, match="built for desktops"):
+                assert create(is_mobile=True) == {"is_mobile": True}
+
+    def test_a_desktop_context_does_not(self):
+        import warnings
+
+        browser = utils.attach_desktop_only_warning(self.FakeBrowser())
+        with warnings.catch_warnings():
+            warnings.simplefilter("error")
+            browser.new_context(viewport={"width": 800, "height": 600}, is_mobile=False)
+            browser.new_page()