@esouthren and @HansMuller, as prior art reference, the use-case and rationale for this is equivalent to the M3 mode of ListTile, that for a brief while had surface color as an opaque background. This made it cumbersome to use it e.g. in popup menus with elevation tint in M3.

ListTile was changed to use transparent background. I think @TahaTesser was involved in that change.

As stated in the issue, the M3 spec says background color of unselected segments is transparent. It is admittedly a bit vague about doing that. The illustrations show them as such, and it specifically only mentions background color of selected segments.

Current implementation of the SegmentedButton thus already returns null as background color for unselected segments, giving them no color, as the spec specifies. The unselected segments do however not become transparent, since the entire button is drawn on opaque Material. Changing it to transparent Material did not break any past tests.

By letting it use transparent Material the SegmentedButton, like ListTile can be used without issue on elevation tinted surfaces.

Making unselected segment buttons fully opaque is still possible by just giving them a color. With this FIX you can also create designs that uses segments in SegmentedButton with partially transparent background on both selected and unselected segments, with different opacities. This is currently not possible, it is a nice feature richness enhancement effect of this tiny FIX.

@rydmike
Can you please rebase to fix google testing?

I've updated the PR, hopefully that will fix the Google Testing "your PR is out of date" problem

auto label is removed for flutter/flutter, pr: 123293, due to - The status or check suite Linux_android android_defines_test has failed. Please fix the issues identified (or deflake) before re-applying this label.

Sorry, 1 am here, was not online, even though I often am at this time :)
Thx for updating the PR Hans. Looks like the Linux_android android_defines_test failed, also after the update.

I took a gamble and just restarted Linux_android android_defines_test, since it didn't appear to have anything to do with this PR.

The failing customer test is https://github.com/macosui/macos_ui/blob/dev/test/selectors/date_picker_test.dart#L292:

| ══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
| The following TestFailure was thrown running a test:
| Expected: <2>
|   Actual: <3>
|
| When the exception was thrown, this was the stack:
| #4      main.<anonymous closure>.<anonymous closure> (file:///tmp/flutter_customer_testing.macos_ui.DOFKBA/tests/test/selectors/date_picker_test.dart:292:11)
| <asynchronous suspension>
| <asynchronous suspension>
| (elided one frame from package:stack_trace)
|
| This was caught by the test expectation on the following line:
|   file:///tmp/flutter_customer_testing.macos_ui.DOFKBA/tests/test/selectors/date_picker_test.dart line 292
| The test description was:
|   Can change the month by clicking the left and right calendar view controls
| ════════════════════════════════════════════════════════════════════════════════════════════════════

auto label is removed for flutter/flutter, pr: 123293, due to - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.

There are a small number of internal golden image test failures (internal google link cl/520561370)

Expected:

Actual:

The change looks OK to me (the button backgrounds are supposed to be transparent).

@esouthren do the diffs look OK to you?

The shown images are as can be seen different. The expected one has a bit of background color and actual one does not, since it is transparent. This would be the case if some color was used on SegmentedButton's underlying canvas Material by default in the expected golden result.

There are a small number of internal golden image test failures (internal google link cl/520561370)

The change looks OK to me (the button backgrounds are supposed to be transparent).

@esouthren do the diffs look OK to you?

LGTM, thanks

@XilaiZhang - this PR is being held up by the same issue as #123294: #123294 (comment). What do we need to do to see the Google Testing results?

temp workaround: the test failures are at test/521658605 and the cl is cl/521658605

Thanks for reporting the issue! will keep track of it in b/260764527