[WIP, not for review] Workaround for setComposition styling clobber

third_party/WebKit/Source/core/editing/InputMethodController.cpp

Index: third_party/WebKit/Source/core/editing/InputMethodController.cpp
diff --git a/third_party/WebKit/Source/core/editing/InputMethodController.cpp b/third_party/WebKit/Source/core/editing/InputMethodController.cpp
index 0bd3b86d91b2380d6a7380e89bc63feae01cc7e0..463af6274d178db1c539603ee1ce81181d7f1886 100644
--- a/third_party/WebKit/Source/core/editing/InputMethodController.cpp
+++ b/third_party/WebKit/Source/core/editing/InputMethodController.cpp
@@ -312,6 +312,39 @@ void InputMethodController::setComposition(const String& text, const Vector<Comp
     // See https://bugs.webkit.org/show_bug.cgi?id=46868
     frame().document()->updateStyleAndLayoutTree();
 
+    // When the IME only wants to change a few characters at the end of the
+    // composition, only touch those characters in order to preserve rich text
+    // substructure.
+    String composing = composingText();
+    bool appending = composing.length() > 0 && composing == text.left(composing.length());
+    bool backspacing = text.length() > 0 && text == composing.left(text.length());

[Changwan Ryu, 2016/09/07 04:14:39]

FYI, this workaround is not going to work for Korean characters (and probably
many more). 

One possible Hangul composition sequence is as follows:
가나다라 -> 가나다람 -> 가나다라마 -> 가나다라맙 -> 가나다라마바

Hangul keyboards consider a consonant to be an ending consonant of the current
character until they see a vowel. And if a vowel follows, the consonant becomes
a starting consonant of a new character.

This is not appending nor backspacing in this case.

Can we extend the logic to actually check for the maximal leftmost (I mean
firstmost) characters?

[aelias_OOO_until_Jul13, 2016/09/07 05:11:49]

OK.  It looks like I can just use std::distance and std::mismatch for this
(http://codereview.stackexchange.com/questions/104477/fastest-c-code-to-get-co...
)

+    if (appending || backspacing) {
+        const EphemeralRange range = compositionEphemeralRange();
+        Element* editable = frame().selection().rootEditableElement();
+        if (!editable)
+            return;
+
+        // Move selection to the end of the composition.
+        PlainTextRange compositionPlainOffsets = PlainTextRange::create(*editable, range);
+        VisibleSelection selection;
+        selection.setWithoutValidation(range.endPosition(), range.endPosition());
+        frame().selection().setSelection(selection, 0);
+        clear();
+
+        // Apply the incremental change.
+        if (appending)
+            confirmCompositionOrInsertText(text.substring(composing.length()), DoNotKeepSelection);
+        else if (backspacing)
+            extendSelectionAndDelete(composing.length() - text.length(), 0);
+
+        // Now recreate the composition starting at its original start, and
+        // apply the specified final selection offsets.
+        setCompositionFromExistingText(underlines, compositionPlainOffsets.start(), compositionPlainOffsets.start() + text.length());
+        selectComposition();
+        setSelectionRangeForSetComposition(selectionStart, selectionEnd);
+        return;
+    }
+
     selectComposition();
 
     if (frame().selection().isNone())
@@ -390,6 +423,24 @@ void InputMethodController::setComposition(const String& text, const Vector<Comp
     if (baseNode->layoutObject())
         baseNode->layoutObject()->setShouldDoFullPaintInvalidation();
 
+    setSelectionRangeForSetComposition(selectionStart, selectionEnd);
+
+    if (underlines.isEmpty()) {
+        frame().document()->markers().addCompositionMarker(m_compositionRange->startPosition(), m_compositionRange->endPosition(), Color::black, false, LayoutTheme::theme().platformDefaultCompositionBackgroundColor());
+        return;
+    }
+    for (const auto& underline : underlines) {
+        unsigned underlineStart = baseOffset + underline.startOffset;
+        unsigned underlineEnd = baseOffset + underline.endOffset;
+        EphemeralRange ephemeralLineRange = EphemeralRange(Position(baseNode, underlineStart), Position(baseNode, underlineEnd));
+        if (ephemeralLineRange.isNull())
+            continue;
+        frame().document()->markers().addCompositionMarker(ephemeralLineRange.startPosition(), ephemeralLineRange.endPosition(), underline.color, underline.thick, underline.backgroundColor);
+    }
+}
+
+void InputMethodController::setSelectionRangeForSetComposition(int selectionStart, int selectionEnd)
+{
     // In case of exceeding the left boundary.
     int selectionOffsetsStart = static_cast<int>(getSelectionOffsets().start());
     int start = std::max(selectionOffsetsStart + selectionStart, 0);
@@ -415,19 +466,6 @@ void InputMethodController::setComposition(const String& text, const Vector<Comp
     const Position& endPosition = endRange.endPosition();
     const EphemeralRange selectedRange(startPosition, endPosition);
     frame().selection().setSelectedRange(selectedRange, TextAffinity::Downstream, SelectionDirectionalMode::NonDirectional, NotUserTriggered);
-
-    if (underlines.isEmpty()) {
-        frame().document()->markers().addCompositionMarker(m_compositionRange->startPosition(), m_compositionRange->endPosition(), Color::black, false, LayoutTheme::theme().platformDefaultCompositionBackgroundColor());
-        return;
-    }
-    for (const auto& underline : underlines) {
-        unsigned underlineStart = baseOffset + underline.startOffset;
-        unsigned underlineEnd = baseOffset + underline.endOffset;
-        EphemeralRange ephemeralLineRange = EphemeralRange(Position(baseNode, underlineStart), Position(baseNode, underlineEnd));
-        if (ephemeralLineRange.isNull())
-            continue;
-        frame().document()->markers().addCompositionMarker(ephemeralLineRange.startPosition(), ephemeralLineRange.endPosition(), underline.color, underline.thick, underline.backgroundColor);
-    }
 }
 
 void InputMethodController::setCompositionFromExistingText(const Vector<CompositionUnderline>& underlines, unsigned compositionStart, unsigned compositionEnd)