[release/10.0] Fix ScriptTagHelper regression by checking for existing content before processing importmaps

[github-actions]

Script tag helper ask mode PR content

Backport of #63155 to release/10.0

/cc @MackinnonBuck @copilot

Fix ScriptTagHelper regression by checking for existing content before processing importmaps

Preserves developer-provided content in the <script> tag helper when its type is "importmap".

Description

In .NET 9, we updated the <script> tag helper to automatically include an import map for fingerprinted JavaScript assets when its type attribute was set to "importmap".

However, if the app had been specifying import map content explicitly, then that content would get overridden by the updated tag helper logic.

This fix changes ScriptTagHelper to preserve the any developer-specified import map content.

Fixes #58973

Customer Impact

This bug impacts customers upgrading to .NET 9 or 10 from .NET 8. The linked issue has notable community engagement which indicates that many customers are affected by this break.

There is a workaround, which is to use the following syntax to bypass the tag helper:

<!script type="importmap">...</!script>

Regression?

• Yes
• No

Regressed in .NET 9

Risk

• High
• Medium
• Low

The fix is straightforward and has been validated via both automated and manual testing.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[danmoseley]

several customers reported this, upgrade impediment from .NET 8, limited risk. approved

[MackinnonBuck]

@wtgodbe, the build is failing because this technically changes the public API of ScriptTagHelper by removing and adding an override. Since this isn't technically a breaking API change, do you think it's fine if we bypass the API baseline check?

[joperezr]

Since this isn't technically a breaking API chang

Was the original API added in 10? If so, is there a way to just fix the api txt files so this doesn't trigger? Separate, but I'm assuming we are going to (if we haven't already) add this to the breaking changes list for RC2?

[MackinnonBuck]

Was the original API added in 10?

The removed override has been shipping since before .NET 10.

Separate, but I'm assuming we are going to (if we haven't already) add this to the breaking changes list for RC2?

I'm pretty sure this isn't a breaking change (even though the API is changing), as the removed override is still defined on the base type. Maybe I'm mistaken about this not being a breaking change, but this suggests it's allowed.