fix(qmltermwidget): scan only written cells and the history before pruning combined characters

qmltermwidget/lib/Character.h

@@ -226,6 +226,7 @@ private:
     // Drops every sequence no screen shows any more, freeing its buffer.
     void pruneUnused();
     static constexpr int kPruneThreshold = 4096;
+    int pruneAt = kPruneThreshold;
     // calculates the hash key of a sequence of unicode points of size 'length'
     uint extendedCharHash(uint* unicodePoints , ushort length) const;
     // tests whether the entry in the table specified by 'hash' matches the

qmltermwidget/lib/Emulation.cpp

@@ -449,7 +449,7 @@ void ExtendedCharTable::pruneUnused()
     {
         if (w->screen())
         {
-            usedExtendedChars += w->screen()->usedExtendedChars();
+            w->screen()->collectUsedExtendedChars(usedExtendedChars);
         }
     }
 
@@ -467,6 +467,10 @@ void ExtendedCharTable::pruneUnused()
             it = extendedCharTable.erase(it);
         }
     }
+
+    // Still-used sequences stay; pruning again only once the table has doubled
+    // keeps a screen full of them from being rescanned on every new one.
+    pruneAt = qMax(kPruneThreshold, int(extendedCharTable.size()) * 2);
 }
 
 uint ExtendedCharTable::createExtendedChar(uint* unicodePoints , ushort length)
@@ -482,7 +486,7 @@ uint ExtendedCharTable::createExtendedChar(uint* unicodePoints , ushort length)
     // The table only ever grew; a long session with many combining
     // sequences keeps every one until the screens are asked which are
     // still in use.
-    if (extendedCharTable.size() >= kPruneThreshold)
+    if (extendedCharTable.size() >= pruneAt)
         pruneUnused();
 
     // check existing entry for match

qmltermwidget/lib/Screen.cpp

@@ -1139,6 +1139,35 @@ void Screen::eraseAboveCursor()
     cuY = 0;
 }
 
+void Screen::collectUsedExtendedChars(QSet<uint> &used) const
+{
+    // Lines grow only as far as the cursor has written, so most are shorter than columns.
+    for (int y = 0; y < lines; ++y)
+    {
+        for (const Character &ch : screenLines[y])
+        {
+            if (ch.rendition & RE_EXTENDED_CHAR)
+                used << ch.character;
+        }
+    }
+
+    // Scrollback still draws its sequences; freeing them would show another one in their place.
+    QVector<Character> cells;
+    for (int line = 0; line < history->getLines(); ++line)
+    {
+        const int length = history->getLineLen(line);
+        if (length <= 0)
+            continue;
+        cells.resize(length);
+        history->getCells(line, 0, length, cells.data());
+        for (const Character &ch : std::as_const(cells))
+        {
+            if (ch.rendition & RE_EXTENDED_CHAR)
+                used << ch.character;
+        }
+    }
+}
+
 int Screen::lastNonBlankLine() const
 {
     const Character blank;

qmltermwidget/lib/Screen.h

@@ -575,22 +575,8 @@ public:
       */
     static void fillWithDefaultChar(Character* dest, int count);
 
-    QSet<uint> usedExtendedChars() const
-    {
-        QSet<uint> result;
-        for (int i = 0; i < lines; ++i)
-        {
-            const ImageLine &il = screenLines[i];
-            for (int j = 0; j < columns; ++j)
-            {
-                if (il[j].rendition & RE_EXTENDED_CHAR)
-                {
-                    result << il[j].character;
-                }
-            }
-        }
-        return result;
-    }
+    /** Adds the extended characters on the screen and in its history to @p used. */
+    void collectUsedExtendedChars(QSet<uint> &used) const;
 
 private:
     Screen(const Screen &) = delete;

tests/vt102emulation_test.cpp

@@ -53,6 +53,15 @@ public:
         return line.at(column);
     }
 
+    // A cell of the oldest line in the history.
+    Character historyCell(int column)
+    {
+        const Screen *screen = m_window->screen();
+        QVector<Character> line(screen->getColumns());
+        screen->getImage(line.data(), line.size(), 0, 0);
+        return line.at(column);
+    }
+
     QString line(int length)
     {
         QString text;
@@ -281,6 +290,36 @@ private slots:
         QCOMPARE(terminal.cell(2).rendition, terminal.cell(1).rendition);
         QVERIFY(terminal.cell(3).foregroundColor == CharacterColor(COLOR_SPACE_RGB, 0xff0000));
     }
+
+    void combiningSequencesInHistorySurviveAPrune()
+    {
+        Terminal terminal;
+        terminal.keepHistory();
+        terminal.print(QStringLiteral("é\r\n").toUtf8());
+        for (int i = 0; i < 60; ++i)
+            terminal.print("\r\n");
+        QVERIFY(terminal.historyLines() > 0);
+
+        // Thousands of distinct sequences written over one short line, each
+        // gone from the screen by the next, push the table past its pruning size.
+        for (int base = 0x100; base < 0x180; ++base) {
+            QString text = QStringLiteral("\r");
+            for (int mark = 0x300; mark < 0x340; ++mark) {
+                text += QChar(base);
+                text += QChar(mark);
+            }
+            terminal.print(text.toUtf8());
+        }
+
+        const Character kept = terminal.historyCell(0);
+        QVERIFY(kept.rendition & RE_EXTENDED_CHAR);
+        ushort length = 0;
+        const uint *chars = ExtendedCharTable::instance.lookupExtendedChar(kept.character, length);
+        QVERIFY(chars);
+        QCOMPARE(length, ushort(2));
+        QCOMPARE(chars[0], uint('e'));
+        QCOMPARE(chars[1], uint(0x301));
+    }
 };
 
 QTEST_MAIN(Vt102EmulationTest)