Add missing inset keyword for inset-shadow-none (#20208)

This PR fixes an issue where the `inset-shadow-none` class did not use
the `inset` keyword. This PR fixes that by introducing the `inset`
keyword:

```diff
  .inset-shadow-none {
-   --tw-inset-shadow: 0 0 #0000;
+   --tw-inset-shadow: inset 0 0 #0000;
    box-shadow: var(--tw-inset-shadow), var
  }
```

While the end effect is the same (there will be no shadow), it also
means that we switch between the types of shadows. This results in the
fact that things like transitions don't behave the way they should.

A minimal reproduction looks like this:
https://play.tailwindcss.com/iOrnKWP49a (depending on when you visit
this link, it might be fixed)

## Test plan

1. Updated tests to reflect
2. Verified in the Vite playground that this is the case. In the video
you will notice that the transition is smooth in the fixed case, but
there is no transition in the current state:



https://github.com/user-attachments/assets/15d8c78e-6be5-4c6c-8df9-1044c14e8f39

CHANGELOG.md

@@ -30,6 +30,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure `@source` globs with symlinks are preserved ([#20203](https://github.com/tailwindlabs/tailwindcss/pull/20203))
 - Ensure later `@source` rules can re-include files excluded by earlier `@source not` rules ([#20203](https://github.com/tailwindlabs/tailwindcss/pull/20203))
 - Upgrade: don't migrate empty class rules to invalid `@utility` rules ([#20205](https://github.com/tailwindlabs/tailwindcss/pull/20205))
+- Ensure transitions between `inset-shadow-none` and other inset shadows work correctly ([#20208](https://github.com/tailwindlabs/tailwindcss/pull/20208))
 
 ### Changed
 

packages/tailwindcss/src/utilities.test.ts

@@ -27850,7 +27850,7 @@ test('inset-shadow', async () => {
     }
 
     .inset-shadow-none {
-      --tw-inset-shadow: 0 0 #0000;
+      --tw-inset-shadow: inset 0 0 #0000;
       box-shadow: var(--tw-inset-shadow), var(--tw-inset-ring-shadow), var(--tw-ring-offset-shadow), var(--tw-ring-shadow), var(--tw-shadow);
     }
 

packages/tailwindcss/src/utilities.ts

@@ -5734,7 +5734,7 @@ export function createUtilities(theme: Theme) {
           if (candidate.modifier) return
           return [
             boxShadowProperties(),
-            decl('--tw-inset-shadow', nullShadow),
+            decl('--tw-inset-shadow', `inset ${nullShadow}`),
             decl('box-shadow', cssBoxShadowValue),
           ]