The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e876485d051f153ac0623ef3197459c6d4d7874a

commit e876485d051f153ac0623ef3197459c6d4d7874a
Author: Katie Dektar <katydek@google.com>
Date: Wed Dec 06 22:49:07 2017

Remove select-to-speak option where search key locks speaking mode.

This "mode" was actually a bug.

Bug:  792557 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I888cd8c3184f42a3dc3c0fdc1c85d747a2e0090f
Reviewed-on: https://chromium-review.googlesource.com/811590
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522232}
[modify] https://crrev.com/e876485d051f153ac0623ef3197459c6d4d7874a/chrome/browser/chromeos/accessibility/select_to_speak_browsertest.cc
[modify] https://crrev.com/e876485d051f153ac0623ef3197459c6d4d7874a/chrome/browser/resources/chromeos/select_to_speak/select_to_speak.js

Actually this isn't fully fixed -- Select-to-Speak is still stealing the mouse movements after the search key is clicked. It won't release them until the search key is clicked a second time. Possibly need to investigate select_to_speak_event_handler.cc.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/679ae71a87843a998741af5e258ab5ac22a22ba4

commit 679ae71a87843a998741af5e258ab5ac22a22ba4
Author: Katie Dektar <katydek@google.com>
Date: Mon Dec 11 18:04:29 2017

Stop capturing events for select-to-speak if the search key was simply tapped.

Bug:  792557 
Change-Id: Ic1ae08f42822a54707b76bcfd54b450f9ef04008
Reviewed-on: https://chromium-review.googlesource.com/815538
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Katie D <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523133}
[modify] https://crrev.com/679ae71a87843a998741af5e258ab5ac22a22ba4/chrome/browser/chromeos/accessibility/select_to_speak_event_handler.cc
[modify] https://crrev.com/679ae71a87843a998741af5e258ab5ac22a22ba4/chrome/browser/chromeos/accessibility/select_to_speak_event_handler_unittest.cc

Google Chrome 67.0.3369.0 (Official Build) canary (64-bit)
Firmware Version Google_Samus.6300.276.0
Flag enabled: #enable-experimental-accessibility-features

Works as expected using these steps:
# Start STS speaking and highlighting text
# tap the search key once
# move mouse around
# start another search + drag selection
# Repeat steps three times in a row 

Works as expected.