Clarification: if mouse moves away from the shelf, a tooltip disappears. However, if mouse moves while staying over the shelf (e.g. the space between the launcher button and the first icon), the tooltip does stay.

It is by design (at least when reading the code) that the tooltip stays while the mouse remains within the shelf.

There are gaps between shelf icons, and we want to show a tooltip continuously (once we had decided to show one) as the mouse moves from one icon to the next. Otherwise one would need to wait the tooltip delay on each icon if they want to see each tooltip.

However, given that the space between the launcher button and the first shelf item can be quite large, it is very debatable whether we should keep showing a tooltip when the mouse is there.

1) If the shelf is quite full and the launcher button is right next to the first shelf item, it seems reasonable not to have an interruption. 2) However if the space is quite large (since items are now centered), it feels very weird to show a tooltip on all this white space.

To me it seems like 2) trumps 1), so I suggest to change things so that a gap is indeed introduced between the launcher/back buttons and the first shelf item.

@Sébastien please let me know if that doesn't sound good to you.

Sounds good to me.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/32fc91bea8effa37436b084d4c5dd30a57157646

commit 32fc91bea8effa37436b084d4c5dd30a57157646
Author: Manu Cornet <manucornet@chromium.org>
Date: Wed Jan 02 18:47:33 2019

CrOS Shelf: Fixes to the tooltip logic

Performance improvements, one small step at a time :-)

The current tooltip logic has at least two issues that this CL
addresses:

* When computing the contiguous area over which to show a tooltip
  continuously, all children of the ShelfView were considered
  (including overflow items for a non-overflow shelf for instance).
* Said contiguous area was recomputed on *every single mouse event*
  over the shelf, thus performing a O(n) (n = # of shelf items)
  operation every mouse move. These are cheap but completely gratuitous
  operations.

Note that this CL will not yet by itself fix the linked bug.

Bug:  896524 
Change-Id: I801bf727232bd588b25dd797fdd20a6a5f675ac3
Reviewed-on: https://chromium-review.googlesource.com/c/1391360
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619442}
[modify] https://crrev.com/32fc91bea8effa37436b084d4c5dd30a57157646/ash/shelf/shelf_view.cc
[modify] https://crrev.com/32fc91bea8effa37436b084d4c5dd30a57157646/ash/shelf/shelf_view.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0748bf4eb6bd22507492bd34312b09af71336509

commit 0748bf4eb6bd22507492bd34312b09af71336509
Author: Manu Cornet <manucornet@chromium.org>
Date: Thu Jan 03 20:00:59 2019

CrOS shelf: Hide tooltip between launcher button and 1st shelf item

Shelf items used to be aligned directly after the launcher button,
so it made sense to include everything within bounds where tooltips
should not hide to avoid hiccups.

However, now that we show centered shelf items, this means that a
tooltip can be shown all over a possibly quite large amount of white
space between the launcher button and the first shelf item.

1) It is very strange to show a tooltip over such a potentially large
piece of shelf. 2) On the other hand, when it does happen that the
first shelf item is close to the launcher button, not showing a
tooltip in the space between them does mean a slight stutter when
hovering from one to the next.

The drawbacks of 1) clearly outweigh the ones of 2), so this change
hides tooltips between the launcher button and the first shelf item.

Bug:  896524 
Change-Id: I06e666ba0296881f87c2cf3ce5972b38d638c3f7
Reviewed-on: https://chromium-review.googlesource.com/c/1393889
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619727}
[modify] https://crrev.com/0748bf4eb6bd22507492bd34312b09af71336509/ash/shelf/shelf_view.cc
[modify] https://crrev.com/0748bf4eb6bd22507492bd34312b09af71336509/ash/shelf/shelf_view_unittest.cc