I tried to take a shortcut here by just selecting everything between the position the selection starts at when TypingCommand::InsertText() and the position the selection starts and ends at after the edit, but this doesn't quite work.

I tried two different approaches:

1. Use a DOM range to mark the position the selection starts at (hoping that it gets updated properly by the edits that occur). This doesn't work because if there's a non-breaking space character and then we insert text immediately afterward with SetComposition(), the nbsp is converted to a regular space and the selection after the edit is expanded to contain the nbsp as a result of the DOM range update (this case is tested in InputMethodControllerTest_SetCompositionWithGraphemeClusterAndMultipleNodes).

2. Use a Position object to record the position. This stays the same across nbsp/space conversions, but there are other cases where the Position becomes disconnected, which makes this problematic.

I think the approach mentioned in the TODO is probably the correct one. I will try that next.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/08e7d08bb2655a8d1d7aafd9db03700492db6722

commit 08e7d08bb2655a8d1d7aafd9db03700492db6722
Author: Ryan Landay <rlanday@chromium.org>
Date: Mon Dec 11 23:17:11 2017

Fix bug where speaking "new paragraph" deletes text

Gboard's voice input feature repeatedly calls InputConnection#setComposingText()
with the in-progress text composition. If you say "new paragraph", it inserts a
pair of newlines into the text. We currently don't handle this case properly
because InputMethodController::SetComposition() calls
TypingCommand::InsertText() (through InsertTextDuringCompositionWithEvents())
with the TypingCommand::kSelectInsertedText flag (which causes InsertText() to
leave the newly-inserted text selected).

InsertText() doesn't actually implement this behavior properly if the text being
composed has a newline in it. It processes the text in multiple "runs,"
inserting line breaks in between. Currently, it inserts the first run, leaving
it selected, then inserts a line break, replacing the first run of text (and
then proceeds similarly for subsequent runs of text).

There's currently a (quite old) FIXME in the code to fix this behavior. I'm
taking a slightly different approach from the one suggested there, and selecting
the inserted text all at once in TypingCommand::InsertText() instead of trying
to modify InsertTextCommand and InsertParagraphSeparatorCommand so they can
extend the current selection. I was inspired by the implementation of
AdjustSelectionAfterIncrementalInsertion() to use a plain text offset from the
start of the editable region to determine the start of the final selection. This
approach is robust to whitespace normalization (this was a problem I ran into
trying to use a DOM range).

Note that with this fix, we no longer need the fix in
https://chromium-review.googlesource.com/c/chromium/src/+/783770 to change the
ending selection in InputMethodController::SetComposition(), as we're no longer
setting the problematic ending selection in TypingCommand::InsertText().

Bug:  784039 
Change-Id: I4b0bb95d1ecf8a979f15b3494f57a0b23e6e2459
Reviewed-on: https://chromium-review.googlesource.com/801979
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523244}
[modify] https://crrev.com/08e7d08bb2655a8d1d7aafd9db03700492db6722/content/public/android/javatests/src/org/chromium/content/browser/input/ImeTest.java
[modify] https://crrev.com/08e7d08bb2655a8d1d7aafd9db03700492db6722/third_party/WebKit/Source/core/editing/commands/TypingCommand.cpp
[modify] https://crrev.com/08e7d08bb2655a8d1d7aafd9db03700492db6722/third_party/WebKit/Source/core/editing/ime/InputMethodControllerTest.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1445f647c0c8d413001c1649ce38490178283774

commit 1445f647c0c8d413001c1649ce38490178283774
Author: Ryan Landay <rlanday@chromium.org>
Date: Mon Dec 11 23:34:24 2017

Remove unused param from TypingCommand::InsertTextRunWithoutNewlines()

The select_insert_text param in this method is only ever called with false after
https://chromium-review.googlesource.com/c/chromium/src/+/801979.

Bug:  784039 
Change-Id: I3c8d0176bfb6600605d42b83676341a61fd83aac
Reviewed-on: https://chromium-review.googlesource.com/820352
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523251}
[modify] https://crrev.com/1445f647c0c8d413001c1649ce38490178283774/third_party/WebKit/Source/core/editing/commands/TypingCommand.cpp
[modify] https://crrev.com/1445f647c0c8d413001c1649ce38490178283774/third_party/WebKit/Source/core/editing/commands/TypingCommand.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e9cfc43960a7229e83b709a76dec3dead5730348

commit e9cfc43960a7229e83b709a76dec3dead5730348
Author: Ryan Landay <rlanday@chromium.org>
Date: Tue Dec 12 01:16:39 2017

Remove select_inserted_text param from Insert[Incremental]TextCommand

After https://chromium-review.googlesource.com/c/chromium/src/+/801979, we no
longer need the select_inserted_text functionality on either InsertTextCommand
or InsertIncrementalTextCommand, so we can remove the parameter.

Bug:  784039 
Change-Id: Icfb2688a83383cea9e295186f351b6f43ffadf6c
Reviewed-on: https://chromium-review.googlesource.com/807286
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523288}
[modify] https://crrev.com/e9cfc43960a7229e83b709a76dec3dead5730348/third_party/WebKit/Source/core/editing/commands/InsertIncrementalTextCommand.cpp
[modify] https://crrev.com/e9cfc43960a7229e83b709a76dec3dead5730348/third_party/WebKit/Source/core/editing/commands/InsertIncrementalTextCommand.h
[modify] https://crrev.com/e9cfc43960a7229e83b709a76dec3dead5730348/third_party/WebKit/Source/core/editing/commands/InsertTextCommand.cpp
[modify] https://crrev.com/e9cfc43960a7229e83b709a76dec3dead5730348/third_party/WebKit/Source/core/editing/commands/InsertTextCommand.h
[modify] https://crrev.com/e9cfc43960a7229e83b709a76dec3dead5730348/third_party/WebKit/Source/core/editing/commands/TypingCommand.cpp