This is a longstanding issue that happens equally when dragging a file by mouse immediately after pressing.
However I think users will notice this issue more often because small icon (=filename) can be hidden behind a finger when using touchscreen, and result in not giving enough visual feedback to the user.

Marianne can take this?

As far as I could tell, using touch it seems to consistently be the thumbnail. Is this maybe tied to specific versions? (Touch).
Will check out clicking.

I did some further testing:
* When dragging immediately after pressing using a mouse, it will start drawing a rectangle in order to select multiple files
* When first selecting and then dragging, it shows the thumbnail.
* I retried getting different results when using touch, but it always seems to show the thumbnail.

Could it be that this bug got fixed since it was reported?

I couldn't reproduce the issue for local files, but
I found for Google drive files, thumbnails are sometimes not shown.

Version:
Google Chrome	62.0.3198.0 (Official Build) dev (64-bit)
Platform	9887.0.0 (Official Build) dev-channel samus

First guess at the cause of the bug: It has to do with being able to load the thumbnail in a specific amount of time.

I'm starting to look through code to find relevant files.

I still see this both for touch and mouse drag on local files also. I'm not sure why it doesn't reproduce. See attached videos.

Chrome 62.0.3199.0
Platform 9897.0.0

Deleted: drag-shadow-mouse.mov 9.1 MB

drag-shadow-mouse.mov 9.1 MB Download

Deleted: drag-shadow-touch.mov 8.9 MB

drag-shadow-touch.mov 8.9 MB Download

Update: at least part of it seems to depend on connectivity.
When checking online files with no connection to the internet, it will always be the filename (probably because it couldn't download the thumbnails). Do we want to touch the behavior in this case? Because that sounds like it is intended to me.

Still hard to reproduce on local files.

Chrome: 62.0.3201.0
Platform: 9900.0.217_09.01.1727

mcirimele@, weifangsun@ which device are you using? It may depend on the processing speed of the device.

I have found the relevant code snippet! (https://cs.chromium.org/chromium/src/ui/file_manager/file_manager/foreground/js/file_transfer_controller.js?l=880&rcl=499d3166266b9c1f3ed10020b2fe81c430c92979). 

It seems like the FileTransferController loads thumbnails on its own rather than using those already fetched by the file list / file grid. Looking into how to use those already fetched by the file list / file grid instead of loading it again - that should solve this issue.

oka@ I've been testing on my Kevin device. Can try to take a look using a Caroline later today. 

I think this may not always happen when using touchscreen, after the recent touch UI update.

We have changed touch interaction around August 15.
- Before the change, dragging by long-tap + slide would have been treated as mouse events.
- After the change, it selects an item upon long press, which is likely before the start of the dragging. So it's more likely to show the thumbnail image in the drag shadow.

Well, when trying to drag&drop with the mouse, you still need to select first and drag then, because if you start by dragging, you will instead draw a rectangle to select multiple files at the same time.

I think oka@ is right with his hint about device processing speed, will do some tests on the Kevin device I have - I usually use the Eve device I have for testing (and that was the one that made it hard to reproduce)

This may be a detail but you don't have to select (check-select) a file before dragging it with mouse. If the row has focus (blue highlight) that is enough. 

Moving this out to M63 given the ongoing investigation

I think there was a bit of misunderstanding here, thanks for clarifying the vocab - what I had meant by "select" before being able to drag&drop was actually focus. The cl adding another way of fetching the thumbnail is currently under review and should be ready by the end of the week.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/900b2cc397ffee541b759ed7ba11297cc454e261

commit 900b2cc397ffee541b759ed7ba11297cc454e261
Author: mariannet <mariannet@google.com>
Date: Wed Sep 13 07:44:08 2017

Use preloaded thumbnails for consistent drag&drop experience.

Sometimes, drag&drop shadows will show thumbnails, and if those haven't
been loaded yet, it will show filenames. However, the thumbnails are
might have been preloaded by other parts of the filesapp, so use these
if possible.
Still keep the old logic as a fall-back.

Bug:  755792 
Tested: manually
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I6a716984bd8854655e4bada2851ef69ab0f7a8b4
Reviewed-on: https://chromium-review.googlesource.com/654499
Commit-Queue: Marianne Thieffry <mariannet@google.com>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Keigo Oka <oka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501564}
[modify] https://crrev.com/900b2cc397ffee541b759ed7ba11297cc454e261/ui/file_manager/file_manager/foreground/js/file_transfer_controller.js
[modify] https://crrev.com/900b2cc397ffee541b759ed7ba11297cc454e261/ui/file_manager/file_manager/foreground/js/ui/file_grid.js
[modify] https://crrev.com/900b2cc397ffee541b759ed7ba11297cc454e261/ui/file_manager/file_manager/foreground/js/ui/file_table.js