Wayland IME: Fix caret position on preedit string.

Currently, there are two issues.
- Delegate method is called after resetting preedit cursor
  so the passed value is always -1.
- According to the protocol definition, it is the byte-offset,
  but we need to convert it to the string16's offset.

Bug: 1140536
Test: Ran manually with Lacros.
Change-Id: I06b018b7e945208f9f1f166ecf0dcf36d882a2c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2486010Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821180}

ui/ozone/platform/wayland/host/wayland_input_method_context.cc

@@ -26,6 +26,13 @@
 #endif
 
 namespace ui {
+namespace {
+
+size_t OffsetFromUTF8Offset(const base::StringPiece& text, uint32_t offset) {
+  return base::UTF8ToUTF16(text.substr(0, offset)).size();
+}
+
+}  // namespace
 
 WaylandInputMethodContext::WaylandInputMethodContext(
     WaylandConnection* connection,
@@ -126,18 +133,13 @@ void WaylandInputMethodContext::SetSurroundingText(
 }
 
 void WaylandInputMethodContext::OnPreeditString(const std::string& text,
-                                                int preedit_cursor) {
-  gfx::Range selection_range = gfx::Range::InvalidRange();
-
-  if (!selection_range.IsValid()) {
-    int cursor_pos = (preedit_cursor) ? text.length() : preedit_cursor;
-    selection_range.set_start(cursor_pos);
-    selection_range.set_end(cursor_pos);
-  }
-
+                                                int32_t preedit_cursor) {
   ui::CompositionText composition_text;
   composition_text.text = base::UTF8ToUTF16(text);
-  composition_text.selection = selection_range;
+  composition_text.selection =
+      (preedit_cursor >= 0) ? gfx::Range(OffsetFromUTF8Offset(
+                                  text, static_cast<uint32_t>(preedit_cursor)))
+                            : gfx::Range::InvalidRange();
   ime_delegate_->OnPreeditChanged(composition_text);
 }
 

ui/ozone/platform/wayland/host/wayland_input_method_context.h

@@ -42,7 +42,8 @@ class WaylandInputMethodContext : public LinuxInputMethodContext,
   void Blur() override;
 
   // ui::ZWPTextInputWrapperClient
-  void OnPreeditString(const std::string& text, int preedit_cursor) override;
+  void OnPreeditString(const std::string& text,
+                       int32_t preedit_cursor) override;
   void OnCommitString(const std::string& text) override;
   void OnDeleteSurroundingText(int32_t index, uint32_t length) override;
   void OnKeysym(uint32_t keysym, uint32_t state, uint32_t modifiers) override;

ui/ozone/platform/wayland/host/zwp_text_input_wrapper.h

@@ -27,6 +27,7 @@ class ZWPTextInputWrapperClient {
   // Called when a new composing text (pre-edit) should be set around the
   // current cursor position. Any previously set composing text should
   // be removed.
+  // Note that the preedit_cursor is byte-offset.
   virtual void OnPreeditString(const std::string& text,
                                int32_t preedit_cursor) = 0;
 

ui/ozone/platform/wayland/host/zwp_text_input_wrapper_v1.cc

@@ -121,8 +121,9 @@ void ZWPTextInputWrapperV1::OnPreeditString(
     const char* text,
     const char* commit) {
   ZWPTextInputWrapperV1* wti = static_cast<ZWPTextInputWrapperV1*>(data);
+  int32_t preedit_cursor = wti->preedit_cursor_;
   wti->ResetInputEventState();
-  wti->client_->OnPreeditString(std::string(text), wti->preedit_cursor_);
+  wti->client_->OnPreeditString(std::string(text), preedit_cursor);
 }
 
 void ZWPTextInputWrapperV1::OnPreeditStyling(