Issue 796815  has been merged into this issue.

nit:
>- Double-tapping the folder open the regular context menu, and I can paste the file too
I think "double tapping" here means "touching with two fingers."

Re #3, yes you are right. tapping with two fingers

This issue does not seem Cast specific.

I think the long-tap should open the context menu near the tapped position instead of the one on the top (=toolbar), because the long-tap originally worked as an alias of right-click. It is also consistent with two-finger tap.
https://chromium-review.googlesource.com/c/chromium/src/+/842326

* How the issue happens
Existing code disables the context menu events triggered by long tap in the file list container, since the time when we introduced touch-specific interactions.
https://chromium-review.googlesource.com/c/chromium/src/+/606068/22/ui/file_manager/file_manager/foreground/js/ui/list_container.js
https://chromium-review.googlesource.com/c/chromium/src/+/625506/8/ui/file_manager/file_manager/foreground/js/ui/list_container.js
The new touch interaction is designed to override the standard behavior of Chrome, by setting special handler and suppressing default behavior of Chrome for some events, like 'contextmenu' event. It was needed, for example, to avoid that long-tap triggers both file selection toggling and the context menu. We didn't treat a long-tap in the padding area specially as we thought context menu by long-tap was not a necessary feature. Now we are changing that behavior.

* How the change 842326 works
The FileTableList is responsible for judging whether the long tap is inside the padding or not.
It then notifies to ListContainer by the custom event when the action should open the context menu, so that 'contextmenu' event right after it is not suppressed.

yamaguchi@: I think this one needs input from PM and UX. Let's wait for their opinions.

Yes, I have examined how it could be fixed, but anyways I will wait the input by those.

Assigning back to yamaguchi@ following up from the discussion via email.

The original issue is partially resolved by introducing paste command to gear menu ( Issue 796815 ) So I'll leave this to M66.
https://chromium-review.googlesource.com/c/chromium/src/+/842326

Just to confirm; the discussion via e-mail agreed that we should show the context menu after long pressing in the folder area, correct?

Yes, that's correct.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d4cea81e005e6e3ec5a55b82c916b7f768657066

commit d4cea81e005e6e3ec5a55b82c916b7f768657066
Author: Sasha Morrissey <sashab@chromium.org>
Date: Tue Mar 06 05:48:32 2018

Re-enabled long-press for context menu while not selecting a file

Bug:  796813 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I7502e76508bace4f4bfe469e56d2c6aa4c0c166f
Reviewed-on: https://chromium-review.googlesource.com/923462
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Commit-Queue: Sasha Morrissey <sashab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541048}
[modify] https://crrev.com/d4cea81e005e6e3ec5a55b82c916b7f768657066/ui/file_manager/file_manager/foreground/js/ui/list_container.js