If you could resize the columns, then the resized columns would be non-useful because all of their text would be elided.

There's an easy solution: resize the window instead. That will make more room There's plenty of screen space in your screenshot.

We can investigate making the default window size bigger and/or increasing its size when the preview is opened rather than having the preview eat into the existing space.

(In reply to Nate Graham from comment #1)
> If you could resize the columns, then the resized columns would be
> non-useful because all of their text would be elided.

They are non-useful from the start, after opening the file dialog. You have Name column too narrow to read file name, while Filesize and Date have convenient width.

https://imgur.com/a/5Y2vUC8

And I can't change the size of Name column to see file names. User is forced to maximize the whole window.

This is highly unusual behavior. For example win32 file picker gives you method for granular control of column widths. 

https://imgur.com/a/bhrjKaK

You can even manage the types of file properties shown in the file dialog:
https://imgur.com/a/BRcxgTe

Gtk file picker gives you method for controling column widths:
https://imgur.com/a/sKwSApr

And the same is for macOS:

https://www.ultraimg.com/image/PlWV

So the KDE file picker is the only one now which has no method for changing the column width. In my opinion it is against UX expected by users.

Step back and ask yourself: "what is the goal?"

The goal is to be able to see all the information in the columns.

Now let's imagine that all the columns were resizable for this scenario. You would resize individual columns such that the name column's items are fully visible... but now the information in other columns is not visible!

By far the better solution to your actual problem is to increase the size of the window instead. You don't need to maximize it, you just need to resize it to make it a bit bigger. As your screenshot shows, there is plenty of space to do this. When your screen is full os unused yet useful space, it's far easier and better to use it rather than trying to micro-optimize a cramped layout in a tiny space.

(In reply to Nate Graham from comment #3)
> Step back and ask yourself: "what is the goal?"
> 
> The goal is to be able to see all the information in the columns.
> 
> Now let's imagine that all the columns were resizable for this scenario. You
> would resize individual columns such that the name column's items are fully
> visible... but now the information in other columns is not visible!

Well, maybe I am not very interested in the content of the other columns? I resize them according to my custom needs.

Removing such possibility is in my opinion against the spirit of KDE/Plasma, definitely more in the line with GNOME/Gtk paradigm, where the UX is forced on the used.

Yet even in the Gtk file dialog, you have option to hide Size column. 
And we have KDE dialog where nothing in this file list view can be customized. 

I just don't see the point of making a default of a unique behavior, going against user habits from Windows, macOS and GNOME/Gtk. Such fixed width for columns should be optional.

(In reply to Adam Golański from comment #4)
> (In reply to Nate Graham from comment #3)
> > Step back and ask yourself: "what is the goal?"
> > 
> > The goal is to be able to see all the information in the columns.
> > 
> > Now let's imagine that all the columns were resizable for this scenario. You
> > would resize individual columns such that the name column's items are fully
> > visible... but now the information in other columns is not visible!
> 
> Well, maybe I am not very interested in the content of the other columns? I
> resize them according to my custom needs.
> 
> Removing such possibility is in my opinion against the spirit of KDE/Plasma,
> definitely more in the line with GNOME/Gtk paradigm, where the UX is forced
> on the used.
> 
> Yet even in the Gtk file dialog, you have option to hide Size column. 
> And we have KDE dialog where nothing in this file list view can be
> customized. 
> 
> I just don't see the point of making a default of a unique behavior, going
> against user habits from Windows, macOS and GNOME/Gtk. Such fixed width for
> columns should be optional.

Then you can use Short View with icons on the side, or Tree View. Both of them don't even show the other columns. Click on the two sliders icon in the toolbar to see these options.

If you can articulate an actual need, we can reconsider. But it seems like we already provide all the functionality you've said you want. Allowing you to resize columns such that their content is elided when there is plenty of room to increase the window size only allows you to make the user interface less useful for yourself, which does not make sense to support. If you don't want to see the other columns at all, you can use a mode that doesn't include them. If you want to use Detailed View or Tree View and customize which columns are visible, that's a valid wish, but unrelated to this because the file dialog has never had that feature in the first place.

Short View doesn't show vertical list, needs to be scrolled horizontally. 
Tree View is something else altogether.

I just don't see any reason for removing the expected behavior, especially in way which makes default view broken – Name column becomes unreadable before window is resized.

It would be nice to remember that KDE used to have normal behavior for this feature – users could easily change column width according to their needs.

https://www.ultraimg.com/image/PqHm

If you don't consider this as a bug, you should at least consider make in default view full-width Name column. Usually name of the file is more important for the user than its size or date. This view is just broken from the usability viewpoint:

https://www.ultraimg.com/image/PqHn

(In reply to Adam Golański from comment #6)
> https://www.ultraimg.com/image/PqHn

When you're in this situation, If you could manually adjust the size of the name column, that would cause the other columns to get cut off. However, currently if you make the window wider, it will *automatically* make the Name column wider too, while preserving the visibility of the other columns. I don't understand why you don't just do this, because it seems like a better solution than resizing the name column and causing the other ones to get cut off. Can you help me understand why you don't want to do it?

(In reply to Nate Graham from comment #7)
> (In reply to Adam Golański from comment #6)
> > https://www.ultraimg.com/image/PqHn
> 
> When you're in this situation, If you could manually adjust the size of the
> name column, that would cause the other columns to get cut off. However,
> currently if you make the window wider, it will *automatically* make the
> Name column wider too, while preserving the visibility of the other columns.
> I don't understand why you don't just do this, because it seems like a
> better solution than resizing the name column and causing the other ones to
> get cut off. Can you help me understand why you don't want to do it?

Maybe I have this file dialog on very small resolution (i.e raspberry pi touchscreen 800x480, or in VMware virtual machine without installed drivers)? Or maybe I need to see things in the background and don't want to resize window to a full screen by clicking "maximize"?

Can you help me understand why the possibility of changing column size was removed from KDE file picker Detailed View? 

However the most important thing is why in the default size, column "Name" becomes unreadable, as shown there. User shall not have to do extra steps (e.g. window resizing) just to see the file names.

https://www.ultraimg.com/image/PqHn

Actually what am I even arguing over!? I've already decided to implement the requested feature in Bug 401506.

Sorry for the noise, will fix.

*** This bug has been marked as a duplicate of bug 401506 ***