| ... | ... | @@ -9565,10 +9565,44 @@ void nsLayoutUtils::ComputeSystemFont(nsFont* aSystemFont, | 
| 9565 | 9565 |    gfxFontStyle fontStyle;
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| 9566 | 9566 |    nsAutoString systemFontName;
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| 9567 | 9567 |    if (aDocument->ShouldResistFingerprinting()) {
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| 9568 |  | -#ifdef XP_MACOSX
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|  | 9568 | +#if defined(XP_MACOSX)
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| 9569 | 9569 |      systemFontName = u"-apple-system"_ns;
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|  | 9570 | +    // Values taken from a macOS 10.15 system.
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|  | 9571 | +    switch (aFontID) {
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|  | 9572 | +      case LookAndFeel::FontID::Caption:
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|  | 9573 | +      case LookAndFeel::FontID::Menu:
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|  | 9574 | +        fontStyle.size = 13;
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|  | 9575 | +        break;
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|  | 9576 | +      case LookAndFeel::FontID::SmallCaption:
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|  | 9577 | +        fontStyle.weight = gfxFontStyle::FontWeight(700);
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|  | 9578 | +        // fall-through
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|  | 9579 | +      case LookAndFeel::FontID::MessageBox:
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|  | 9580 | +      case LookAndFeel::FontID::StatusBar:
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|  | 9581 | +        fontStyle.size = 11;
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|  | 9582 | +        break;
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|  | 9583 | +      default:
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|  | 9584 | +        fontStyle.size = 12;
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|  | 9585 | +        break;
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|  | 9586 | +    }
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|  | 9587 | +#elif defined(XP_WIN) || defined(MOZ_WIDGET_ANDROID)
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|  | 9588 | +    // Windows uses Segoe UI for Latin alphabets, but other fonts for some RTL
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|  | 9589 | +    // languages, so we fallback to sans-serif to fall back to the user's
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|  | 9590 | +    // default sans-serif. Size is 12px for all system fonts (tried in an en-US
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|  | 9591 | +    // system).
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|  | 9592 | +    // Several Android systems reported Roboto 12px, so similar to what Windows
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|  | 9593 | +    // does.
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|  | 9594 | +    systemFontName = u"sans-serif"_ns;
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|  | 9595 | +    fontStyle.size = 12;
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| 9570 | 9596 |  #else
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|  | 9597 | +    // On Linux, there is not a default. For example, GNOME on Debian uses
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|  | 9598 | +    // Cantarell, 14.667px. Ubuntu Mate uses the Ubuntu font, but also 14.667px.
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|  | 9599 | +    // Fedora with KDE uses Noto Sans, 13.3333px, but it uses Noto Sans on
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|  | 9600 | +    // GNOME, too.
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|  | 9601 | +    // In general, Linux uses some sans-serif, but its size can vary between
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|  | 9602 | +    // 12px and 16px. We chose 15px because it is what Firefox is doing for the
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|  | 9603 | +    // UI font-size.
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| 9571 | 9604 |      systemFontName = u"sans-serif"_ns;
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|  | 9605 | +    fontStyle.size = 15;
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| 9572 | 9606 |  #endif
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| 9573 | 9607 |    } else if (!LookAndFeel::GetFont(aFontID, systemFontName, fontStyle)) {
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| 9574 | 9608 |      return;
 |