Also, there seems to be some incorrect event location or selection range info on Gerrit:
1) Visit https://chromium-review.googlesource.com/c/chromium/src/+/941733/11/ui/views/controls/views_text_services_context_menu_mac.mm#102
2) Double-click the word 'widget' to select that text on line 102
3) Right click the selected word 'widget' and select 'Look Up "Widget"'.
Expected: The popup points to 'widget' with some info for that word.
Actual: The popup points to 'tToTar' off to the left of 'widget' (see attached).

Deleted: Screen Shot 2018-03-07 at 6.54.16 PM.png 286 KB

	Screen Shot 2018-03-07 at 6.54.16 PM.png 286 KB View Download

You might want to start by looking into this method:

https://cs.chromium.org/chromium/src/content/browser/renderer_host/render_widget_host_view_mac.mm?rcl=0d6b1f56c51176f08cec49cb304389545909bc4f&l=1236

The Appkit method showDefinitionForAttributedString:atPoint: gets called with the selected string and the baseline point of the selection. In a multiline selection, the baseline point is at the bottom of the lowest line of text. This is why the dictionary overlay appears in the incorrect location when multiple lines of text are selected.

Is there a way to introduce a new method that would calculate the basepoint for the first line of text?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/da71e35a7c1f7afce21a17dcda49ce30d7890de4

commit da71e35a7c1f7afce21a17dcda49ce30d7890de4
Author: Tessa Nijssen <tnijssen@google.com>
Date: Mon Jul 30 18:33:11 2018

[Mac] Look Up Overlay Appears on First Line

Currently, when a user selects multiple lines of text, right clicks, and
selects "Look Up", the yellow overlay appears on the last line of the
selected text but with the text of the first line of the selected text.

WebSubstringUtil::GetBaselinePoint() was modified to call
FirstRectForRange() instead of ComputeTextRect(). FirstRectForRange()
returns the IntRect for the first line of text which is needed to get
the baseline point of the first line of text.

WebViewTest.WebSubstringUtilBaselinePoint was added to test that the
baseline point does not change when multiple lines of text are selected
compared to a single line of text.

Bug:  819864 
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: Iec0bac8d45baf2142ea5e826ebdfd7d997e7f584
Reviewed-on: https://chromium-review.googlesource.com/1103448
Commit-Queue: Tessa Nijssen <tnijssen@google.com>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Sarah Chan <spqchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579094}
[modify] https://crrev.com/da71e35a7c1f7afce21a17dcda49ce30d7890de4/third_party/blink/renderer/core/editing/web_substring_util.mm
[modify] https://crrev.com/da71e35a7c1f7afce21a17dcda49ce30d7890de4/third_party/blink/renderer/core/exported/web_view_test.cc
[add] https://crrev.com/da71e35a7c1f7afce21a17dcda49ce30d7890de4/third_party/blink/renderer/core/testing/data/content_editable_multiline.html