Ignore an If-Modified-Since that is not a date, instead of failing the request - #897

Description

Fixes #876, and it is not about styles: standalone in the end — that mode is just what makes the request happen.

The trace in the report lands in shelf_static:

GET /main.css
Error on line 1, column 4: Invalid HTTP date: expected " ".
1 │ Thu,27 Aug 2026 20:33:22 GMT
package:http_parser/src/http_date.dart 61         parseHttpDate
package:shelf/src/request.dart 77                 Request.ifModifiedSince
package:shelf_static/src/static_handler.dart 182  _handleFile

shelf_static reads If-Modified-Since through Request.ifModifiedSince, which throws a FormatException on anything it cannot parse. So a client sending a date without the space after the weekday takes the request down with a 500 and an error dump, and main.css never arrives. That also explains the two shapes the reporter sees — right after a reload, and then now and then on a refresh or from another browser: it happens whenever that client has a cached copy and sends the header back.

RFC 9110 §13.1.3 is explicit that a recipient must ignore an If-Modified-Since field value that is not a valid date. So it is dropped and the file is served as if it had not been sent.

Measured against the real shelf_static, with the exact value from the report:

Two places go through it: jaspr_cli's development proxy, which is what serves the generated styles and what the report hits, and the framework's own static handler in server_handler.dart, which has the same exposure for anything it serves out of web/.

What it does not fix

The second half of the report — main.css being generated after the server reloads — is untouched. It is a sequencing question between the css generation step and the server reload, and worth its own issue if it still bites once the 500 is gone.

Type of Change

• 🛠️ Bug fix

Tests

packages/jaspr/test/server/conditional_headers_test.dart, three cases against a file handler that reads the header the way shelf_static does: the value from the report is dropped and the file is served, a valid value is passed through and parses to the right instant, and no header stays no header.

dart test -p vm is green in packages/jaspr (184) and dart test test/helpers in packages/jaspr_cli (24).

Fixes #876