Raising the priority of this one for M63... fukino@ - Can you assign to the right eng on the team?

In the current implementation, 1) 'Click on a thumbnail' and 2) 'Ctrl+Click on a file's row (incl. thumbnail).' have the save effect.
Both operations toggle the file selection.

Should we modify the behavior only for 'Click on a thumbnail'?
In this case, which behavior should we choose for 'Ctrl+Click on a thumbnail'?  (I guess it should be treated as pattern 2?)

Interesting, I didn't know about the CTRL+click on a file row to select pattern. But right now if you have no row focused and you CTRL+click it focuses instead of selecting the row. That seems inconsistent. 

I propose:
- CTRL+click on a row (including thumbnail) acts as a "shortcut" to select independent of start state of that row. 
- Selecting a file clears focus if a different file had focus. This changes behavior for both "click on a thumbnail" and "CTRL+click anywhere on a row). 

I've written up all the cases I can think of in https://docs.google.com/document/d/1lAtmq2uYAV6nhjwhmj_HSm1iFEr3_gqertm-r45K9AY/edit under "Select / deselect behavior". Please comment there if there are cases I haven't covered or if you have questions. 

Based on a discussion on go/files-check-select, we decided to have following two modifications.

* If Files app enters check-select mode by clicking an item's icon, all existing selection should be cleared and the clicked icon should be selected.
* If number of selected items becoms 1 by Ctrl+Click, Files app should enter check-select mode.

I'll make patches for these modifications.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/305341a844902eb487110785ca8e3a98b14723df

commit 305341a844902eb487110785ca8e3a98b14723df
Author: Naoki Fukino <fukino@chromium.org>
Date: Wed Oct 11 10:56:22 2017

Clear previous selection when entering check-select mode by clicking icon.

When no item is selected and item A's icon is clicked
  => A should be selected.
When A is selected in single-select mode and B's icon is clicked.
  => A should be unselected and B should be selected.
When A is selected in check-select mode and B's icon is clicked.
  => A should stay selected and B should also be selected.

Bug:  769019 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I8a7bd6cfedbd87cdb7eedf9a8b132a728c0f8705
Reviewed-on: https://chromium-review.googlesource.com/711822
Reviewed-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507944}
[modify] https://crrev.com/305341a844902eb487110785ca8e3a98b14723df/ui/file_manager/file_manager/foreground/js/ui/file_table_list.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/be40400e2f2c19fea7aed40e64d470f0dd5814af

commit be40400e2f2c19fea7aed40e64d470f0dd5814af
Author: Naoki Fukino <fukino@chromium.org>
Date: Wed Oct 11 11:43:12 2017

Enter check-select mode when a file is selected by Ctrl+Click.

Wnen a user select a file by Ctrl+Click, it should be reasonable to assume that
the user start interacting multiple files.
Files app should enter check-select mode in such cases.

Bug:  769019 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I64eda17d1096e6b33370a3278cc1a3d896393817
Reviewed-on: https://chromium-review.googlesource.com/711780
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507953}
[modify] https://crrev.com/be40400e2f2c19fea7aed40e64d470f0dd5814af/ui/file_manager/file_manager/foreground/js/ui/file_table_list.js