`_background_supports_alpha` should be added to WinForm ProgressBar

[ikus060]

Describe the bug

As title explain, ProgressBar widget on Windows doesn't support alpha color and raise an exception when set to transparent. To avoid this, the widget class should declare _background_supports_alpha = False

Steps to reproduce

Simply create a Progress bar with TRANSPARENT background.

Expected behavior

Should not raise an exception.

Screenshots

No response

Environment

• Operating System: Windows
• Python version: 3.10
• Software versions:
  • Briefcase: NA
  • Toga: 0.4.2
  • ...

Logs

    self._applicator.set_background_color(value)
  File "c:\Users\vmtest\Documents\minarca-agent\.venv\lib\site-packages\toga\style\applicator.py", line 51, in set_background_color
    self.widget._impl.set_background_color(color)
  File "c:\Users\vmtest\Documents\minarca-agent\.venv\lib\site-packages\toga_winforms\widgets\base.py", line 132, in set_background_color
    self.native.BackColor = win_color
System.ArgumentException: Control does not support transparent background colors.
   at System.Windows.Forms.Control.set_BackColor(Color value)

Additional context

No response

[freakboy3742]

Thanks for the report. Your analysis seems entirely correct; a patch would be welcome.

It looks like this wasn't picked up because the testbed test_progressbar doesn't import any of the color tests (cf the testbed tests for test_textinput. Adding the extra imports will exercise the color APIs; there may be some changes needed on the various platform probes to make the tests pass.

It also makes me wonder how many other widgets aren't exercising these tests...

[ikus060]

@freakboy3742 Thanks for the confirmation. I might try to make a PR for this little change. Question for you. How can I run the testbed ? I don't see any documentation about how to run the test.

[freakboy3742]

@ikus060 There's a section on running the testbed in the contribution guide.