Autocomplete: account for halfway processed composing characters.

Languages that rely on composing characters occasionally include halfway
composed characters in the user input length.

(cherry picked from commit 59eafc5)

Change-Id: Ie5bf92050bfa4c39bc7e7be9b23b763d2b7fb51e
Fixed: 1317406
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4133715
Reviewed-by: Gang Wu <gangwu@chromium.org>
Commit-Queue: Gang Wu <gangwu@chromium.org>
Auto-Submit: Tomasz Wiszkowski <ender@google.com>
Cr-Original-Commit-Position: refs/heads/main@{#1088404}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4162955
Commit-Queue: Tomasz Wiszkowski <ender@google.com>
Cr-Commit-Position: refs/branch-heads/5481@{#238}
Cr-Branched-From: 130f3e4-refs/heads/main@{#1084008}

chrome/browser/ui/android/omnibox/java/src/org/chromium/chrome/browser/omnibox/SpannableAutocompleteEditTextModel.java

@@ -579,7 +579,15 @@ public boolean onBeginImeCommand() {
             if (mBatchEditNestCount == 1) {
                 mPreBatchEditState.copyFrom(mCurrentState);
             } else if (mDeletePostfixOnNextBeginImeCommand > 0) {
+                // Note: in languages that rely on character composition, the last incomplete
+                // character may not be recognized as part of the string, but it may still be
+                // accounted for by the mDeletePostfixOnNextBeginImeCommand.
+                // In such case, the text below is actually shorter than the user input, and the
+                // computed string boundaries enter negative index space.
                 int len = mDelegate.getText().length();
+                if (mDeletePostfixOnNextBeginImeCommand > len) {
+                    mDeletePostfixOnNextBeginImeCommand = len;
+                }
                 mDelegate.getText().delete(len - mDeletePostfixOnNextBeginImeCommand, len);
             }
             mDeletePostfixOnNextBeginImeCommand = 0;