I think it's a longstanding problem, but I don't know if we've ever come up with a workaround. Componenting to the collective.

Chrome Version: 55.0.2883.87 m
OS: Windows 7 64-bit (fully up to date)
Note: I run dual monitors and tested this with one monitor enabled and both enabled. The problem exists in both scenarios. Also, I do not run any taskbar modifying software/add-ons.

What steps will reproduce the problem?
(1) Set the taskbar to auto-hide.
(2) Maximize Chrome.
(3) View any full screen video within Chrome (tested with YouTube and Vimeo).
(4) Return video to partial screen mode.
(5) Mouse to bottom of screen.

What is the expected result?
Taskbar shows.

What happens instead?
Taskbar is not revealed.

Temporary solution:

NOTE: This must be reset every time a full screen video is viewed within Chrome.

(1) Right click taskbar.
(2) Select properties.
(3) Change "Taskbar location on screen" from current setting to any other setting and click apply (e.g. Current setting is 'Bottom' and can be changed to 'Left').
(4) Revert change to "Taskbar location on screen" back to former setting and click apply (Note: You do not have to disable and re-enable auto-hide for this fix, it can be left enabled).

I have been experiencing this problem for at least a year and simply use this temporary fix or utilize another browser for video viewing. After reading many of these threads, I realized no one had provided clear steps for reproducing the issue and thought I would chime in. Please let me know if you need any more details about my computer hardware/software or log files.

Additional answers regarding the notes from mgiuca@chromium.org:

(1) I am not running any task modifying add-ons or software.
(2) I tested this again with Chrome as default and not default (Firefox defaulted in that case) and the problem still persists while not existing in Firefox.
(3) I used task manager to close the explorer process and to start a new explorer process. This did not fix the problem.

The easiest fix is to close and re-open Chrome, but that is not optimal for many users so they must use the temporary solution I listed in the comment above.

Reproduced on fully up to date Windows 10 and Chrome (63.0.3239.132 (Official Build) (64-bit)).

Chrome is the only application that causes this -- it's something in your code.  

If you are not able to repro/diagnose/test this long standing bug, surely you could just always leave enough space that you don't block the taskbar?

This is broken for me as well, on a newly installed Windows 10 box.

BTW, this issue might be related to or even the same as https://bugs.chromium.org/p/chromium/issues/detail?id=152241.

This "issue" has been bugging and annoying me for over year. I have finally found the cause and the solution, 100% reproducibility. Pay attention:

1. It only happens when Chrome restores itself to new max. screen dimensions of the Windows Primary screen, e.g. after sleeping and undocking laptop from work with two extra DisplayPort screens(one of which is Windows Primary) and unsleeping at home with just the laptop own screen.

2. Chrome *apparently* is in Windows maximized state, but it is not! It is only *resized* to the max. screen dimensions; the maximize window icon is still available to prove this.

3. In this resized state, the Windows hidden taskbar (at right for me) will not unhide when touching the screen border with the mouse.

4. Now click the maximize button and observe: visually nothing changes in the Chrome window, however the hidden Windows taskbar will now unhide.

5. Switch some times the Chrome window between Maximized and Restored state and observe each time the change of behaviour of the hidden Windows taskbar.

Is this a Chrome "bug/feature" or is a Windows issue?

Come on, nobody? This is reproducible on every vanilla Windows 10 machine, mine or anyone's. 
Just try my steps and you'll see.