Currently when positioned in its horizontal layout the Tag Panel fields are rather awkwardly flowed when enough custom fields have been added in the options.
The first two columns of tag fields seem to be fixed in position while only the third column subsequently expands downward, which leaves a lot of unused space and requires the panel to be resized very high, at the expense of space in the file view.
What would be great is if fields from columns could flow into other columns depending on the height of the Tag Panel, and also be able to flow into more than three columns. Examples:
Modifications like that have been already suggested in the past. I am sure someone will dig up some old post aiming precisely at what you propose here and now
But there were also other ideas by one and the same user:
And here is what new option I would I like to see: ability to place a chosen field all throughout the whole horizontal space
Imagine this: one long TITLE or ALBUM field that could display all the extremely long names, without the need to resize vertically placed Tag Panel or keeping a wide Column to accommodate those few very long cases. Or LYRICS or whatever you need / like
I am using the horizontal tag-panel since years because the vertical tag-panel made it necessary to scroll in order to see the whole content and I sort of arranged with the insufficiencies of the horizontal layout. I appreciate any approvement but I think for an real approvement it would be necessary to make the layout of the tag-panel really user-configurable, because a definition of the width and the placing of the tag-fiels would really allow to get more tagfields in the tag-panel without the need of scrolling.
The new layout of the horizontal tag-panel places the tag fields on the surface a little better (resulting in a larh´ger horizontal space) but because also the display of the cover is much larger than before and much larger than in the vertical tag-panel I have to scroll again horizontally because I mostly don't work with MP3Tag in a full-windows-screen-modus.
I have no real objection against a larger display of the cover-image but together with the limited possibility of arranging the tag-fields it results for my configuration the need to scroll horizontally.
The main idea is to get as much tagfields in the tag-panel as possible and not to waste space.
To achieve this it would be necessary for the user to have the ability to take influence on the width and the position of the tag-fields.
I have some self-defined tag-fields in the tag-panel that in extreme only need space for 1 character (only a little bit more because of the headline) but without the possibility to have real influence on the position and width they cover much more place than necessary. They could easily be positioned together in 1 row and 1 column like the arrangement of year, track and genre in the original layout.
Normally I don't work with Mp3Tag in full screen mode because I often have to drag and drop or have a look at windows from other applications during my work. I have a 27"-Monitor with a resolution of 2560x1440 and have set windows to a scale of 125 %.
With the new layout of the tag-panel I have to stretch the mp3tag-windows to about 2/3 of the screen horizontally to see the whole tag-panel without the need of scrolling. This is only necessary because when I stretch the windows horizontally the tag-fields in the tag-panel get wider and wider, even those that don't draw any beneficial of a greater width.
To see the resolution of the embedded coverart aside the picture I have to scroll or enlarge the mp3tag-window to 2/3 of the window. And I like to have a view of this resolution permanently.
With the new layout came a larger display of the embedded coverart than before. To have a closer look at the cover-art it is surely better to have this larger display than before. On the other side this new size of the coverart makes it necessary for me to have a wider size of the tag-panel vertically to have a complete view of the cover-art. I think in the past I made a suggestion to have the possibility to blow-up the embedded coverart with a mouse-cliock too, like it it possible at the moment with the separate cover-art in the folder. This would bring much more advantage (have a look at the details) for the user than a slightly larger cover-art in the tag-panel.
I understand that the previous layout with the vertical overflowing tag-fields in the last column wasn't an ideal thing. On the other side I got accustomed to it and a positioning of the tag-fields that I needed not so often at the end of the list made the disadvantage of the need to scroll vertically sometimes not so hard because I had the beneficial to see the cover-art and the most used tag-fields permanently.
Summary:
Every user has special workflows and preferences and every rigid layout will have disadvantages and advantages for someone. So I think only a real flexible layout will overcome these disadvantages.
For my personal workflow the old layout-limitations were much more acceptable than the new one in spite of the circumstance that the old one did only use part of the possible usable space.
Edit:
My explanations to the need of horizontal scrolling and the necessity to enlarge to 2/3 of the screen to avoid this refer to my tag-panel-layout with 4 rows and a multi-line-lyrics-field.
Noticed this was taken into consideration in v2.88. Thanks for remembering, Florian
Just some thoughts on in the way it's implemented currently: it appears the fields don't flow to fill the columns depending on the height of the Tag Panel. So while they will now expand into more than 3 columns (which is great) there seems to be a fixed maximum number of fields/height for each column.
Saw a new topic appear in which the OP used the tag panel at a smaller width previously (with vertical scrollbars) and they wondered if there's a way of restoring the previous layout (at least from what I can glean from translation).
Perhaps a way to reconcile the two concepts would be to always expand fields downward if there's not enough width to expand into a new column (ie: avoiding horizontal scrollbars). Just an idea I figured I'd suggest after seeing that topic.
Not sure how tricky creating such fields that flow to fit the height of the Panel and other columns as necessary would be, while also only expanding downward if there's not enough Mp3Tag window width to add an additional column, but seems like it would suit both workflows. Anyone else care to share further thoughts on this?
Edit:
To illustrate the concept more visually I mocked up some examples in the browser that I took screenshots of, each with a progressively wider Tag Panel. The dark grey bar on the right represents a scrollbar.
Many thanks @LaurenBacall for the good ideas and the screen mocks. I'm also still thinking about the best wayTM to address this and I can definitely see that the current approach still has a lot of limitations (I definitely like it more than the endless third column in the previous version).
My preferred way of imaging the future horizontal layout currently has those properties:
Use vertical space (no fixed column break after n columns)
Use horizontal space (no fixed max column width)
Shrink fields horizontally if this prevents the horizontal scrollbar (to a certain min column width)
This would result in a layout that
Never (?) shows a vertical scrollbar
Only shows a horizontal scrollbar, if the columns and the cover art window cannot be arranged in the visible space.
So it's a combination of your last screen mock and something that is not shown yet. Mocks 1, 3 and 4 would not be possible with this approach.
Edit: I just remembered what I was thinking about this morning. I could invent a special Tag Panel element that serves as a column break. This way, the precedence of height can be overwritten and the column break happens only at those manual breaks. A vertical scrollbar is then possible, which enable more narrow layouts with many fields that don't show a horizontal scrollbar.
Flexible width fields seem like a decent idea. A drawback with that would be some common fields like Artist and Comments might get too narrow at certain window sizes to be as easily usable as they are currently with the fixed width fields but I'd assume you'd make them some appropriate minimum width.
Not sure how a horizontal-only overflow would be received by users when there's no available room to shrink the fields (since wouldn't that also hide the cover art area until scrolled into view?) but sounds like from your edit that you've found a solution which is good to hear.
If there are any beta versions in the future of this concept I'd certainly be glad to test them
I had "The week of the Tag Panel" I've rewritten the Tag Panel layout (again) and added the following improvements with the current Development Build Mp3tag v2.88c:
Tag Panel fields now fill up the vertical space and flow horizontally. If you want to exceed the available vertical space, you need to manually add _SEPARATOR fields where column breaks are needed.
Added context menu to customize fields,
Added option to specify size when adding fields (normal, smaller, small, multiline),
Added option to enable/disable fields.
Changed all Tag Panel fields to be customizable (incl. default fields)
It's clear that this needs thorough testing and I'm looking forward to your feedback
This is incredible. Not only have you addressed the prior concerns but added long requested features like the ability to change the length of fields and hide defaults, among other things.
When you first mentioned separators wasn't sure how well it would work but after using it I think it probably addresses the issue for those preferring vertical scrolling over horizontal for overflows well, since it's so customizable and the user can force however many or few columns they wish. (Also since field lengths can be shortened it leaves room for more in the same space). Well done.
One main observation so far:
It looks like the Smaller Size of field field length is currently exactly half the width of the Normal field length but seems to not account for the margin in-between which makes them not stack side-by-side when followed by another Smaller field. I'm assuming they're meant to be able to do so like the Small length fields can but for two fields instead of three?
Thanks for the quick fix These new changes honestly make a horizontal layout so enjoyable, and love the added touches like scaling the cover art in size depending on the panel height and adding user-customizable Alt key hotkeys to any field
What do you think I should do with regard to marking the topic 'Solved'? I'm thinking of marking your post as the solution but would you prefer I wait until a non-beta release?
Thought I'd also capture some GIFs to demonstrate the auto-flowing fields without separators, and how fixed columns look with separators, for those just reading along.
I am pretty impressed with the new sizing feature.
Even though I use a pretty standard vertical layout, setting small fields like LANGUAGE or BPM to a small layout really makes the tag panel look much nicer. And it saves space.
I've rewritten the Tag Panel layout (again) and added the following improvements with the current Development Build 
