Bug 1544455 - part1 : check the base direction of cue text in 'ComputedPositionAlign()'. r=heycam

According to the spec [1], when text alignment is `start` or `end`, we have to check the base direction of the cue text in order to decide the position alignment.

[1] https://www.w3.org/TR/webvtt1/#cue-computed-position-alignment

Differential Revision: https://phabricator.services.mozilla.com/D35270

--HG--
extra : moz-landing-system : lando

dom/media/TextTrackCue.cpp

@@ -9,6 +9,7 @@
 #include "mozilla/dom/TextTrackRegion.h"
 #include "nsComponentManagerUtils.h"
 #include "mozilla/ClearOnShutdown.h"
+#include "unicode/ubidi.h"
 
 extern mozilla::LazyLogModule gTextTrackLog;
 
@@ -205,10 +206,24 @@ PositionAlignSetting TextTrackCue::ComputedPositionAlign() {
     return PositionAlignSetting::Line_left;
   } else if (mAlign == AlignSetting::Right) {
     return PositionAlignSetting::Line_right;
+  } else if (mAlign == AlignSetting::Start) {
+    return IsTextBaseDirectionLTR() ? PositionAlignSetting::Line_left
+                                    : PositionAlignSetting::Line_right;
+  } else if (mAlign == AlignSetting::End) {
+    return IsTextBaseDirectionLTR() ? PositionAlignSetting::Line_right
+                                    : PositionAlignSetting::Line_left;
   }
   return PositionAlignSetting::Center;
 }
 
+bool TextTrackCue::IsTextBaseDirectionLTR() const {
+  // The returned result by `ubidi_getBaseDirection` might be `neutral` if the
+  // text only contains netural charaters. In this case, we would treat its
+  // base direction as LTR.
+  return ubidi_getBaseDirection(mText.BeginReading(), mText.Length()) !=
+         UBIDI_RTL;
+}
+
 void TextTrackCue::NotifyDisplayStatesChanged() {
   if (!mReset) {
     return;

dom/media/TextTrackCue.h

@@ -298,6 +298,8 @@ class TextTrackCue final : public DOMEventTargetHelper {
   void SetDefaultCueSettings();
   nsresult StashDocument();
 
+  bool IsTextBaseDirectionLTR() const;
+
   RefPtr<Document> mDocument;
   nsString mText;
   double mStartTime;