Use len(draw_data.cmd_lists) in imgui backend (#830)

* Draw a frame in test_examples_run

<details><summary>Claude's draft</summary>

test_examples_run() ran an example's __main__ block and stopped there. Under
the offscreen canvas that only constructs the canvas and registers a draw
function, so nothing is drawn and the draw function -- the bulk of most
examples -- was never executed. Draw one frame for every example that exposes
a canvas.

Drawing is not enough on its own: rendercanvas calls the draw function inside
its log_exception() context, so an exception there is logged to the
"rendercanvas" logger and swallowed, and a frame still comes out. Assert that
nothing was logged there.

This makes the imgui examples fail against imgui_bundle 1.92.900, which is the
bug reported in #829 and is fixed in
the next commit.

Resume this Claude session:
```
cd /home/mark/git/wgpu-py
claude --resume 590dd7bc-312c-419e-ad3d-2c69d00943cd
```
</details>

* Use len(draw_data.cmd_lists) in imgui backend

<details><summary>Claude's draft</summary>

`ImDrawData.cmd_lists_count` was a legacy alias, documented in the imgui_bundle
stubs as `== CmdLists.Size`, and imgui_bundle removed it in 1.92.900. Every
`ImguiWgpuBackend.render()` call raises AttributeError against that release.
Fixes #829.

`len(draw_data.cmd_lists)` is exactly what the removed attribute equalled, and
works across the whole supported `imgui-bundle>=1.92.0,<2` range: verified
against 1.92.0, 1.92.3, 1.92.4, 1.92.5, 1.92.600, 1.92.601, 1.92.700, 1.92.801
and 1.92.900. No shim or version check is needed, and the pin does not need
narrowing.

The test from the previous commit fails without this change and passes with it.

Resume this Claude session:
```
cd /home/mark/git/wgpu-py
claude --resume 590dd7bc-312c-419e-ad3d-2c69d00943cd
```
</details>

Author: hmaarrfk

CHANGELOG.md

@@ -33,6 +33,10 @@ Possible sections in each release:
   Downstream code should capture that error and skip the frame, optionally invoking some sort of sleep to save energy.
   See https://github.com/pygfx/wgpu-py/pull/820 for context.
 
+### Fixed:
+* The imgui backend no longer uses ``ImDrawData.cmd_lists_count``, which was removed in imgui-bundle 1.92.900.
+  See https://github.com/pygfx/wgpu-py/issues/829.
+
 
 ## [v0.31.1] - 23-06-2026
 

examples/tests/test_examples.py

@@ -4,6 +4,7 @@
 
 import os
 import importlib
+import logging
 import runpy
 import sys
 from unittest.mock import patch
@@ -177,11 +178,24 @@ def update_diffs(module, is_similar, img, stored_img, *, atol):
 
 
 @pytest.mark.parametrize("module", examples_to_run)
-def test_examples_run(module, force_offscreen):
+def test_examples_run(module, force_offscreen, caplog):
     """Run every example marked to see if they can run without error."""
-    # use runpy so the module is not actually imported (and can be gc'd)
-    # but also to be able to run the code in the __main__ block
-    runpy.run_module(f"examples.{module}", run_name="__main__")
+    with caplog.at_level(logging.ERROR, logger="rendercanvas"):
+        # use runpy so the module is not actually imported (and can be gc'd)
+        # but also to be able to run the code in the __main__ block
+        module_globals = runpy.run_module(f"examples.{module}", run_name="__main__")
+
+        # The main block only sets up the canvas and hands it a draw function;
+        # the offscreen canvas does not draw until asked. So ask, otherwise the
+        # draw function (the bulk of most examples) is never executed.
+        canvas = module_globals.get("canvas")
+        if canvas is not None:
+            canvas.draw()
+
+    # rendercanvas calls the draw function inside a log_exception() context, so
+    # errors in it do not propagate; they only show up in the log.
+    errors = [r for r in caplog.records if r.name == "rendercanvas"]
+    assert not errors, caplog.text
 
 
 if __name__ == "__main__":

wgpu/utils/imgui/imgui_backend.py

@@ -445,7 +445,7 @@ def render(
         fb_width = int(display_width * draw_data.framebuffer_scale.x)
         fb_height = int(display_height * draw_data.framebuffer_scale.y)
 
-        if fb_width <= 0 or fb_height <= 0 or draw_data.cmd_lists_count == 0:
+        if fb_width <= 0 or fb_height <= 0 or len(draw_data.cmd_lists) == 0:
             return
 
         if draw_data.textures is not None: