Fix regression of isBlockImportant()

isBlockImportant() was relying strictly on the hash bits
to detect whether a matching filter was `important`, but
this approach regressed with changes with how `important`
filters are compiled. This commit fixed this by no longer
relying on the hash bits but rather on an internal
register variable being set by `important` filters when
they match.

I couldn't find any actual cases in default filter lists
(including a couple of default regional lists) that the
regression is having any effect, due to the limited cases
for which isBlockImportant() is called.

A test was added in a previous commit to detect such
regression in the future:
- a76935b

src/js/static-net-filtering.js

@@ -195,6 +195,7 @@ let $docDomain = '';
 let $tokenBeg = 0;
 let $patternMatchLeft = 0;
 let $patternMatchRight = 0;
+let $isBlockImportant = false;
 
 const $docEntity = {
  entity: undefined,
@@ -298,9 +299,6 @@ class LogData {
  isRegex: false,
  };
  filterUnits[iunit].logData(logData);
- if ( (categoryBits & Important) !== 0 ) {
- logData.options.unshift('important');
- }
  if ( (categoryBits & ThirdParty) !== 0 ) {
  logData.options.unshift('3p');
  } else if ( (categoryBits & FirstParty) !== 0 ) {
@@ -605,7 +603,7 @@ registerFilterClass(FilterTrue);
 
 const FilterImportant = class {
  match() {
- return true;
+ return ($isBlockImportant = true);
  }
 
  logData(details) {
@@ -3522,6 +3520,14 @@ class FilterCompiler {
  this.action |= HEADERS;
  }
 
+ // Important
+ //
+ // IMPORTANT: must always appear at the end of the sequence, so as to
+ // ensure $isBlockImportant is set only for matching filters.
+ if ( (this.optionUnitBits & this.IMPORTANT_BIT) !== 0 ) {
+ units.push(FilterImportant.compile());
+ }
+
  // Modifier
  //
  // IMPORTANT: the modifier unit MUST always appear first in a sequence
@@ -3545,7 +3551,6 @@ class FilterCompiler {
  // meant to override.
  if ( (this.action & ActionBitsMask) === BlockImportant ) {
  this.action &= ~Important;
- units.push(FilterImportant.compile());
  this.compileToAtomicFilter(
  FilterCompositeAll.compile(units),
  writer
@@ -4259,6 +4264,7 @@ FilterContainer.prototype.matchRequestReverse = function(type, url) {
  $requestURL = urlTokenizer.setURL(url);
  $requestURLRaw = url;
  $requestTypeValue = (typeBits & TypeBitsMask) >>> TypeBitsOffset;
+ $isBlockImportant = false;
  this.$filterUnit = 0;
 
  // These registers will be used by various filters
@@ -4327,11 +4333,13 @@ FilterContainer.prototype.matchRequest = function(fctxt, modifiers = 0) {
  $docEntity.reset();
  $requestHostname = fctxt.getHostname();
  $requestTypeValue = (typeBits & TypeBitsMask) >>> TypeBitsOffset;
+ $isBlockImportant = false;
 
  // Evaluate block realm before allow realm, and allow realm before
  // block-important realm, i.e. by order of likelihood of a match.
  const r = this.realmMatchString(BlockAction, typeBits, partyBits);
  if ( r || (modifiers & 0b0010) !== 0 ) {
+ if ( $isBlockImportant ) { return 1; }
  if ( this.realmMatchString(AllowAction, typeBits, partyBits) ) {
  if ( this.realmMatchString(BlockImportant, typeBits, partyBits) ) {
  return 1;
@@ -4538,7 +4546,7 @@ FilterContainer.prototype.toLogData = function() {
 /******************************************************************************/
 
 FilterContainer.prototype.isBlockImportant = function() {
- return (this.$catBits & ActionBitsMask) === BlockImportant;
+ return this.$filterUnit !== 0 && $isBlockImportant;
 };
 
 /******************************************************************************/