@justinmc This PR replaces #159328 as the workaround that was used in #159328 is no more needed since #159975 was merged.

Many thanks for the feedback 🙏 .
Based on #163205 (comment). I will mark this PR as draft, file a separate PR to make the inputDecoration prefix and suffix gaps configurable. And then come back to this PR. Especially trying to understand in which cases "the suffix is very close to the right edge now".

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Here's what I meant by the suffix icon being close to the right side:

The master branch	Your screenshot of this PR in the description	This PR currently

You can see in the PR currently the suffix icon is further to the right than it was before you changed the PR.

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Center(
              child: SizedBox(
                width: 200,
                child: DropdownButtonFormField<String>(
                  onChanged: (_) {},
                  decoration: InputDecoration(
                    fillColor: Colors.blueGrey.withOpacity(0.2),
                    filled: true,
                    labelText: 'Label text',
                  ),
                  items: [
                    DropdownMenuItem(
                      value: 'selected',
                      child: Text('Selected item'),
                    ),
                  ],
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

Wow, thanks a lot for providing this sample and the screenshot.
For the moment, I'm surprised that the screenshot in my PR description did not match the result you got. I will try to find why there is such a discrepancy (maybe related to another InputDecoration parameter or maybe related to the platform used to run the sample). Anyway, thanks a lot for providing these very useful feedback 🙏

@justinmc I updated the PR based on your last comment and our discussion on #167558 (comment)

It turned out that the fix, in order to compute the correct suffixIconConstraints, should take into account if the input decoration is filled or outlined.

Because for a filled or outlined decoration the right padding is directly provided by the constraints added to the suffix icon, for instance constraints on mobile are 48x48 which means a left and right padding of 12.0. (FYI, on desktop it is be 8.0 because the suffixIconConstraints is 40x40, this is probably not intended as from the M3 spec, visual density usually applies only on the vertical direction, I will check that and file an issue if needed).

I updated the fix accordingly. Let me know how it goes with the Google tests.
FYI, I will need to add some more tests for verifying this fix.

And let me try to explain the layout:

Before the fix

DropdownButtonFormField was positioning the icon by setting the input decoration content to a Row containing both the selected item label and the icon (24x24).

For a filled or outlined decoration, the horizontal layout was: leftPadding (16.0) | selected item label | icon (24.0) | rightPadding (16.0).
(16.0 padding since very recently because it is due to #162157, before it was 12.0).

For a non-filled and non-outlined decoration, it was: leftPadding (0.0) | selected item label | icon (24.0) | rightPadding (0.0)

With the fix

DropdownButtonFormField is positioning the icon by setting the input decoration suffixIcon and suffixIconConstraints.

For a filled or outlined decoration, the horizontal layout is: leftPadding (16.0) | selected item label | suffixIconGap (4.0) | icon (24.0) | rightPadding (12.0).
Here the right padding is 12.0 and this is the compliant padding from the M3 spec: when there is no suffix icon right padding should be 16.0, when the is a suffixIcon it should be 12.0).
This means that, with the fix, the content's width is the same as before this fix. The difference is the icon moving by 4.0 pixel to the right (which will restore its position as before #162157).

For a non-filled and non-outlined decoration, it is: leftPadding (0.0) | selected item label | icon (24.0) | suffixIconGap (4.0) | rightPadding (0.0)
Here the difference will be the content width being reduced by 4.0 pixel and the icon should be at the same horizontal position.

Here is a combined screenshot to illustrate this. The top 3 fields showcase the fix, the bottom ones were captured without the fix:

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  final bool filled = true;
  static const List<DropdownMenuItem<String>> items = [
    DropdownMenuItem(value: 'selected', child: Text('Selected item')),
  ];

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: SizedBox(
            width: 200,
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              spacing: 4.0,
              children: [
                DropdownButtonFormField<String>(
                  onChanged: (_) {},
                  decoration: const InputDecoration(labelText: 'Raw'),
                  // Before https://github.com/flutter/flutter/pull/163205, width could be 176.
                  selectedItemBuilder:
                      (context) => [Container(height: 24, width: 172, color: Colors.amber)],
                  items: items,
                ),
                DropdownButtonFormField<String>(
                  onChanged: (_) {},
                  decoration: const InputDecoration(labelText: 'Filled', filled: true),
                  selectedItemBuilder:
                      (context) => [Container(height: 24, width: 144, color: Colors.amber)],
                  items: items,
                ),
                DropdownButtonFormField<String>(
                  onChanged: (_) {},
                  decoration: const InputDecoration(
                    labelText: 'Outlined',
                    border: OutlineInputBorder(),
                  ),
                  selectedItemBuilder:
                      (context) => [Container(height: 24, width: 144, color: Colors.amber)],
                  items: items,
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

Nice, I ran it locally and confirmed that the layout seems to be correct as you explained. The Google test check has disappeared though, let me try re-adding it.

Edit: Possibly an infrastructure problem (Google internal b/416058821) so I'm running the Google tests manually.