Prevent the IME candidate window from flickering on Chrome OS

ui/views/controls/textfield/textfield_model.cc

Index: ui/views/controls/textfield/textfield_model.cc
diff --git a/ui/views/controls/textfield/textfield_model.cc b/ui/views/controls/textfield/textfield_model.cc
index 392abf88c361944ef18c7e991dd2a7edf842c437..f4d2282c55a72775bf2f779e8ad7748e9dcf2b72 100644
--- a/ui/views/controls/textfield/textfield_model.cc
+++ b/ui/views/controls/textfield/textfield_model.cc
@@ -282,6 +282,23 @@ void SetKillBuffer(const base::string16& buffer) {
   *kill_buffer = buffer;
 }
 
+void SelectRangeInCompositionText(gfx::RenderText* render_text,
+                                  size_t cursor,
+                                  const gfx::Range& range) {
+  DCHECK(render_text);
+  DCHECK(range.IsValid());
+  uint32_t start = range.GetMin();
+  uint32_t end = range.GetMax();
+#if defined(OS_CHROMEOS)
+  // Swap |start| and |end| so that GetCaretBounds() can always return the same
+  // value during conversion.
+  // TODO(yusukes): Check if this works for other platforms. If it is, use this
+  // on all platforms.
+  std::swap(start, end);
+#endif
+  render_text->SelectRange(gfx::Range(cursor + start, cursor + end));

[oshima, 2017/05/22 09:55:21]

what's the reason why we don't change the UI side to align the window to left
instead?

[Yusuke Sato, 2017/05/22 16:55:02]

That's another option, but strictly speaking, if we do that, the new behavior
won't match chrome.input.ime API document at
https://developer.chrome.com/extensions/input_ime#type-WindowPosition which
explicitly says that the window position follows the cursor, not the selection
range. We could add yet another WindowPosition type like "selection" and let the
Japanese IME migrate to the new mode, but this seems overkill given that the
Blink side (RWHVA) already implements the existing "cursor" mode very well.

So instead of changing the UI behavior, this CL tried to improve Textfield as
good as RWHVA regarding the "cursor" mode support. WDYT?

+}
+
 }  // namespace
 
 using internal::Edit;
@@ -656,13 +673,10 @@ void TextfieldModel::SetCompositionText(
     // range (or caret position) in the composition here we use a selection-like
     // marker instead to show this range.
     // TODO(yukawa, msw): Support thick underlines and remove this workaround.
-    render_text_->SelectRange(gfx::Range(
-        cursor + emphasized_range.GetMin(),
-        cursor + emphasized_range.GetMax()));
+    SelectRangeInCompositionText(render_text_.get(), cursor, emphasized_range);
   } else if (!composition.selection.is_empty()) {
-    render_text_->SelectRange(gfx::Range(
-        cursor + composition.selection.GetMin(),
-        cursor + composition.selection.GetMax()));
+    SelectRangeInCompositionText(render_text_.get(), cursor,
+                                 composition.selection);
   } else {
     render_text_->SetCursorPosition(cursor + composition.selection.end());
   }