Once directory with multiple albums/audiobooks is open would be great to be able to click on one mp3 file and then without need to use mouse have a keyboard shortcut e.g. Alt+A to select all files which reside in the same folder as currently selected file.
Step 1 select file
Step 2 press Alt+A
Result all files residing in this folder selected.
Have you considered to use the already existing function of filters?
I think that Alt-A opens the actions menu (at least in the German version)
This feature is needed for speed of operation, rather than filtering, Alt+A is just example it can be any hotkey.
I have 100 audiobooks loaded and going trough them downloading "Tag Sources" from Audible book by book.
It works relatively fast when i use keyboard only, only slowdown in the process is need to use mouse to select the range of files files for certain audiobook.
Of course i can use keyboard Shift+Arrow up/down to select however there's still a degree of "aiming" that needs to be done to not include file from next audio book in the range.
Did you know that you can drag&drop files and folders from the Windows Explorer to MP3tag? So perhaps there is an alternative to loading all files at once.
To get a comprehensive impression of the function:
The precondition is: ALBUM is filled, all files for 1 album are in the same directory.
What should happen if you press the hotkey but the precondition is not met and ALBUM is not filled and/or there are files for several albums in the same directory and/or the files for the same album are still scattered across several directories?
In my opinion dragging and dropping each book/folder is even slower (because need to grab the mouse, switch windows, aim, select, drag, wait for files to load). Purpose of "hotkey" is so there would be not need to touch mouse.
I don't see why ALBUM or any other tag needs to be filled. We can call "hotkey" - "select all files within path"
logic:
What if step 2 would involve clicking on the respective grid column label or the tag info panel field label while holding down one of Ctrl, Alt or Shift?
An easy way would be: Double click on a file in the _folderpath column -> select all files in the same folder. Next double click would select all files from parent folder. And so on.
But the OP request a hotkey, no mouse action.
For a keyboard shortcut it feels more complicated in the grid view, because unless you’re editing a field there, a whole row is selected at all times, whereas the mouse pointer can easily focus a specific column as well.
Can make both ways Alt + click and Alt+T or any other to select no issue.
I think its good feature to have some checkbox in interface or shortcut Alt + click to select whole folder on mouse click.
This way user open multiple folders at once and work not by selecting individual file, but rather selecting one file and it automatically selects folder with all files belonging to the folder.
Little background on why this is feature is desired by me. When i need to preform action on the whole album (in 90% of cases i use ID3tag to modify whole album, not individual files) drag and drop individual albums is extra work and selecting files from one album in the list requires focusing my mind - reading aiming mouse (to indecently not include file from next or previous album) which drains my mind of energy quick especially when i need to make routine procedure like getting tags from web, for 100+ albums.
I like that very much also. If all you need is double click "Path " column and it selects all files in that path. Even though it requires grabbing mouse, but also also takes out a lot of unnecessary brain work.
Perhaps can still allow to open mp3 file in player by double clicking "Filename" column.
All 3 methods of selection can use same exact code to execute, only difference is in the way that code gets path to the folder either by:
That is possible, like this:
Make sure "Select files automatically" has a check-mark (in General options).
Create this tool:
Make sure it is one of the first nine tools in the Tools list. Let's say it is number 3.
Select 1 file in Mp3tag.
Now Ctrl + 3 (because it is the third tool) will highlight all the files contained in the directory of the selected file.
Note:
Files that are not already in the list will be added to the bottom of the list.
Works like charm thanks.
You can define 10 instead of nine tools with a number. 1 - 0
Uh oh when i used this method more, i find one annoying problem the list of files always scrolls down and first file in selected folder becomes last one in the list.
Its sort of okay if i go trough list of albums starting from the bottom of list, but just for the sake of having it right, is there a workaround?
Yes, ryerman's solution works. Unfortunately, it is not a solution for me: it means file import, and in such cases the list focus jumps to first column. (This is the only case, where the phenomenon has survived, at least in my configuration.)
It sounds as if a file that is contained in the target folder was not in the original list.
My method does not directly select the files in a folder.
Instead, it adds files that are presumably already in the list and takes advantage of how Mp3tag does that.
Try sorting the list by path before you start work.
That guarantees that all files in a folder are shown together.
Now, if after using the tool, some files appear at the bottom of the list, they were not in the list when you started.
https://help.mp3tag.de/main_cli.html
When files are sorted by path, and automatic file selection is on, this is what I see:
It's hard to say if it works right or wrong.
I don't understand the implications of your complaint, but maybe this will help you find a work-around.
Before using selection tool
After using selection tool
This is still usable when you go trough entire list starting from bottom going up, but what i need for mp3tag don't scroll the list upon selection, just highlight the files.
I tried very hard to reproduce the behaviour you describe, without success.
I played around with some options and even copied the file structure that you have.
But the only scrolling I see is as I described in my previous post.
My results:
Before using selection tool
After using selection tool
There must be something different between what you and I have and/or do, but I can't figure out what it is. 
What operating system you have and what version of mp3tag? I see yo use different layout could it be a reason why.
I am using portable version of MP3Tag i uploaded my entire mp3tag folder with settings here if you want to try.
Good idea. 
I uninstalled my Mp3tag and tried your program files.
The tool then worked as you have shown.
The difference is between the portable installation and the standard installation.
If you switch to the standard installation, I predict that the tool will work as I have described.
I can't explain the difference. Probably Florian can.
fyi, my system details: Windows 10 Home, 64 bit, Mp3tag v3.08c
(I show them as my "Name", which you can see by clicking on my icon.)
I do not have this option selected - and yet your hefty tool seems to be working A-OK
Thus this tool should ot be named
Select All Files In Folder
but
Add All Files From Folder
[just to be precise]
