The icons are being displayed using background-image, selecting 1x or 2x for whatever the device matches best. When we've got a 1.5x device, the bitmap gets scaled, resulting in aliasing artifacts that look blurry. See the attached Zip and open 'index.html'. The 9 rows at the bottom are 3 rows of 3 icons, in each one the middle row is the 1.5x case - on the left the 1x icon scaled up, on the right the 2x icon scaled down. These bitmap assets should be replaced by SVG assets so we can get proper display at any screen density.

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Hi, the 3-dot icons svg can be found here:
https://icons.googleplex.com/#icon=ic_more_vert&search=more

You can use the 24dp svg for the 1.5x case. 

The CL in here: https://chromium-review.googlesource.com/c/chromium/src/+/1343401 replaces the 3 icons visible in selected mode with SVG files that scale. Attached are 2 images, first the FilesApp with no zooming, the second zoomed in twice, both show clean edges on the icons.

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The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/483c269dfb6b32fcde61652e4c02d9ce38ede6d8

commit 483c269dfb6b32fcde61652e4c02d9ce38ede6d8
Author: Alex Danilo <adanilo@chromium.org>
Date: Tue Nov 20 04:58:18 2018

Fix blurry icons for devicePixelRatio 1.5

When files are in selection mode, the icons on the action bar
use scaled bitmaps, originally supplied as 1x and 2x assets.
Zooming the window also blurs the edges due to scaling.
Replaced the PNGs for these icons with SVG files.

Change-Id: Id3b31d6ce909f7da5839916271c48f93e7a23bd0
Bug:  793382 
Test: Manually inspect and zoom window and check for blur
Reviewed-on: https://chromium-review.googlesource.com/c/1343401
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609598}
[modify] https://crrev.com/483c269dfb6b32fcde61652e4c02d9ce38ede6d8/ui/file_manager/file_manager/foreground/css/file_manager.css
[add] https://crrev.com/483c269dfb6b32fcde61652e4c02d9ce38ede6d8/ui/file_manager/file_manager/foreground/images/files/ui/delete.svg
[add] https://crrev.com/483c269dfb6b32fcde61652e4c02d9ce38ede6d8/ui/file_manager/file_manager/foreground/images/files/ui/menu.svg
[add] https://crrev.com/483c269dfb6b32fcde61652e4c02d9ce38ede6d8/ui/file_manager/file_manager/foreground/images/files/ui/share.svg

Done: https://chromium-review.googlesource.com/c/chromium/src/+/1343401

Follow-on bug needs to be logged to change the other PNGs to SVGs, dependent on UI refresh work in progress

What icon sizes are we using for the replacement?
The original icons used in the current version of the files app are 16*16px. The new ones looks way bigger in comparison. 

In addition, what is their rendering in 1x? They were created in .png in 1x and 2x to obtain proper rendering at low scale. If we want to use .svg, they need to be created on the base 1x icon grid and scaled up, not from a 24*24px  .svg.

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Good point, they were 20px, I've modified the assets to 16px grid and will be uploading them in a new CL now.