LibGfx+LibWeb: Only trigger @font-face loads from text shaping

Author: https://github.com/jdahlin
Commit: https://github.com/LadybirdBrowser/ladybird/commit/8c49029bb76
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/9032
Reviewed-by: https://github.com/Psychpsyo

Libraries/LibGfx/FontCascadeList.cpp

@@ -58,20 +58,27 @@ void FontCascadeList::extend(FontCascadeList const& other)
     m_pending_faces.extend(other.m_pending_faces);
 }
 
-Gfx::Font const& FontCascadeList::font_for_code_point(u32 code_point) const
+Gfx::Font const& FontCascadeList::font_for_code_point(u32 code_point, TriggerPendingLoads trigger_pending_loads) const
 {
-    // Walk pending entries first: if this codepoint falls in an unloaded face's
-    // unicode-range we kick off the fetch and drop the entry — a fallback font that
-    // happens to cover the codepoint shouldn't prevent the real face from loading.
-    // FontComputer::clear_computed_font_cache() rebuilds the cascade once the fetch
-    // completes, so later shapes pick up the loaded face. Run before the ASCII cache
-    // lookup so a previously-cached codepoint still triggers a newly-added face.
-    m_pending_faces.remove_all_matching([code_point](auto const& pending) {
-        if (!pending->covers(code_point))
-            return false;
-        pending->start_load();
-        return true;
-    });
+    // Only the text-shaping paths pass TriggerPendingLoads::Yes. Probes that don't
+    // lead to a glyph being drawn (the U+0020 check used to compute first-available-
+    // font metrics, for instance) skip this block so they can't initiate a download
+    // for a subset face that happens to cover the probe codepoint.
+    if (trigger_pending_loads == TriggerPendingLoads::Yes) {
+        // Walk pending entries first: if this codepoint falls in an unloaded face's
+        // unicode-range we kick off the fetch and drop the entry — a fallback font
+        // that happens to cover the codepoint shouldn't prevent the real face from
+        // loading. FontComputer::clear_computed_font_cache() rebuilds the cascade
+        // once the fetch completes, so later shapes pick up the loaded face. Run
+        // before the ASCII cache lookup so a previously-cached codepoint still
+        // triggers a newly-added face.
+        m_pending_faces.remove_all_matching([code_point](auto const& pending) {
+            if (!pending->covers(code_point))
+                return false;
+            pending->start_load();
+            return true;
+        });
+    }
 
     if (code_point < m_ascii_cache.size()) {
         if (auto const* cached = m_ascii_cache[code_point])

Libraries/LibGfx/FontCascadeList.h

@@ -43,7 +43,15 @@ public:
 
     void extend(FontCascadeList const& other);
 
-    Gfx::Font const& font_for_code_point(u32 code_point) const;
+    // A pending-face fetch should only be initiated for codepoints that are actually
+    // being shaped into glyph runs. Callers that merely probe the cascade (e.g. the
+    // U+0020 check in "first available font" metrics) pass No so that probing does
+    // not kick off downloads for subset faces that happen to cover the probe point.
+    enum class TriggerPendingLoads : u8 {
+        No,
+        Yes,
+    };
+    Gfx::Font const& font_for_code_point(u32 code_point, TriggerPendingLoads = TriggerPendingLoads::No) const;
 
     bool equals(FontCascadeList const& other) const;
 

Libraries/LibGfx/TextLayout.cpp

@@ -121,7 +121,7 @@ Vector<NonnullRefPtr<GlyphRun>> shape_text(FloatPoint baseline_start, Utf16View
 
     auto it = string.begin();
     auto substring_begin_offset = string.iterator_offset(it);
-    Font const* last_font = &font_cascade_list.font_for_code_point(*it);
+    Font const* last_font = &font_cascade_list.font_for_code_point(*it, FontCascadeList::TriggerPendingLoads::Yes);
     FloatPoint last_position = baseline_start;
 
     auto add_run = [&runs, &last_position, letter_spacing](Utf16View const& string, Font const& font) {
@@ -132,7 +132,7 @@ Vector<NonnullRefPtr<GlyphRun>> shape_text(FloatPoint baseline_start, Utf16View
 
     while (it != string.end()) {
         auto code_point = *it;
-        auto const* font = &font_cascade_list.font_for_code_point(code_point);
+        auto const* font = &font_cascade_list.font_for_code_point(code_point, FontCascadeList::TriggerPendingLoads::Yes);
         if (font != last_font) {
             auto substring = string.substring_view(substring_begin_offset, string.iterator_offset(it) - substring_begin_offset);
             add_run(substring, *last_font);

Libraries/LibWeb/Layout/TextNode.cpp

@@ -643,7 +643,7 @@ Gfx::Font const& TextNode::ChunkIterator::font_for_space(size_t at_index, u32 sp
     for (size_t i = at_index; i < m_view.length_in_code_units();) {
         auto cp = m_view.code_point_at(i);
         if (!is_interword_space(cp) && cp != '\t' && cp != '\n') {
-            auto const& font = m_font_cascade_list.font_for_code_point(cp);
+            auto const& font = m_font_cascade_list.font_for_code_point(cp, Gfx::FontCascadeList::TriggerPendingLoads::Yes);
             if (!font.is_emoji_font() && has_glyph(font))
                 return font;
             // Text is coming from an emoji face; we'll fall back to (3).
@@ -653,7 +653,7 @@ Gfx::Font const& TextNode::ChunkIterator::font_for_space(size_t at_index, u32 sp
     }
 
     // 3. No text around (leading/trailing/all spaces) — pick a font with the glyph from the cascade.
-    return m_font_cascade_list.font_for_code_point(space_code_point);
+    return m_font_cascade_list.font_for_code_point(space_code_point, Gfx::FontCascadeList::TriggerPendingLoads::Yes);
 }
 
 Optional<TextNode::Chunk> TextNode::ChunkIterator::next_without_peek()
@@ -680,7 +680,7 @@ Optional<TextNode::Chunk> TextNode::ChunkIterator::next_without_peek()
     auto const& expected_font_for = [&](u32 cp) -> Gfx::Font const& {
         return is_interword_space(cp)
             ? font_for_space(m_current_index, cp)
-            : m_font_cascade_list.font_for_code_point(cp);
+            : m_font_cascade_list.font_for_code_point(cp, Gfx::FontCascadeList::TriggerPendingLoads::Yes);
     };
 
     auto const& font = expected_font_for(current_code_point());

Tests/LibWeb/Text/expected/css/font-face-probe-does-not-trigger-load.txt

@@ -1 +1 @@
-ProbeFont status: error
+ProbeFont status: unloaded