style: Only apply grayscale filter for printing on print preview.

Differential Revision: https://phabricator.services.mozilla.com/D88065

components/style/gecko/media_features.rs

@@ -521,6 +521,18 @@ fn eval_moz_is_glyph(
     query_value.map_or(is_glyph, |v| v == is_glyph)
 }
 
+fn eval_moz_print_preview(
+    device: &Device,
+    query_value: Option<bool>,
+    _: Option<RangeOrOperator>,
+) -> bool {
+    let is_print_preview = device.is_print_preview();
+    if is_print_preview {
+        debug_assert_eq!(device.media_type(), MediaType::print());
+    }
+    query_value.map_or(is_print_preview, |v| v == is_print_preview)
+}
+
 fn eval_moz_is_resource_document(
     device: &Device,
     query_value: Option<bool>,
@@ -601,7 +613,7 @@ macro_rules! system_metric_feature {
 /// to support new types in these entries and (2) ensuring that either
 /// nsPresContext::MediaFeatureValuesChanged is called when the value that
 /// would be returned by the evaluator function could change.
-pub static MEDIA_FEATURES: [MediaFeatureDescription; 55] = [
+pub static MEDIA_FEATURES: [MediaFeatureDescription; 56] = [
     feature!(
         atom!("width"),
         AllowsRanges::Yes,
@@ -799,6 +811,12 @@ pub static MEDIA_FEATURES: [MediaFeatureDescription; 55] = [
         Evaluator::Ident(eval_moz_os_version),
         ParsingRequirements::CHROME_AND_UA_ONLY,
     ),
+    feature!(
+        atom!("-moz-print-preview"),
+        AllowsRanges::No,
+        Evaluator::BoolInteger(eval_moz_print_preview),
+        ParsingRequirements::CHROME_AND_UA_ONLY,
+    ),
     system_metric_feature!(atom!("-moz-scrollbar-start-backward")),
     system_metric_feature!(atom!("-moz-scrollbar-start-forward")),
     system_metric_feature!(atom!("-moz-scrollbar-end-backward")),

components/style/gecko/media_queries.rs

@@ -206,6 +206,15 @@ impl Device {
         self.reset_computed_values();
     }
 
+    /// Returns whether this document is in print preview.
+    pub fn is_print_preview(&self) -> bool {
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            None => return false,
+        };
+        pc.mType == structs::nsPresContext_nsPresContextType_eContext_PrintPreview
+    }
+
     /// Returns the current media type of the device.
     pub fn media_type(&self) -> MediaType {
         let pc = match self.pres_context() {