`@media print` throwing an error after upgrading from LESS 4.4.0 to 4.4.1

[kgetz-arista]

To reproduce:

Upgrade from 4.4.0 to 4.4.1.

Current behavior:

10:26:32  [error] ✗ Build failed in 15.95s
10:26:32  [error] error during build:
10:26:32  [vite:css-post] [lightningcss minify] Unexpected token Function("print")
10:26:32  
10:26:32  12984|    flex-direction: row;
10:26:32  12985|  }
10:26:32  12986|  @media print(max-width: 992px) {
10:26:32     |        ^
10:26:32  12987|    .main-view {
10:26:32  12988|      display: flex;
10:26:32  
10:26:32      at minifyCSS

Expected behavior:

Vite build should work as usual.

Environment information:

• less version: 4.4.1
• nodejs version: 22.14.0
• operating system: Windows 11, Linux

[dosubot]

Hi @kgetz-arista! I'm Dosu and I’m helping the less.js team.

This error happens because LESS 4.4.1 enforces stricter CSS media query syntax. The parser now requires a space between the media type and the feature, so @media print(max-width: 992px) is invalid and should be written as @media print and (max-width: 992px). Without the space and the and, LESS emits CSS like print(max-width: 992px), which downstream tools (like lightningcss) interpret as a function call, causing the build to fail with an "Unexpected token Function('print')" error.

To fix this, update your LESS to use the standard syntax: @media print and (max-width: 992px) or @media (print) and (max-width: 992px) reference.

To reply, just mention @dosu.

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[kgetz-arista]

Our original code has an OR (not an AND), so this isn't quite correct. Here's our code which is being incorrectly transformed now:

@media print, (max-width: 992px) {

It appears that this is being wrongly transformed to this:

@media print(max-width: 992px) {

[kgetz-arista]

Glancing at the changelog for this release, maybe #4349 is the reason for the change in behavior? @puckowski

[abradley]

This issue can be reproduced using the Less-to-CSS playground for version 4.4.1. With Less version 4.3.0, the output is correct.

[puckowski]

@kgetz-arista Thank you for the bug report. Surprised the tests missed the regression, but this should be resolved by #4363

I am short on time today but I'll add more tests to #4363 tomorrow and hopefully we can get it merged soon.

[kgetz-arista]

Thanks for the quick fix! 🚀