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Vim Popups

Taken from this stackexchange answer.

The answer, given by user 'B Layer', in the above link was so clear and complete, I wanted to preserve it here. The full answer has image examples and also covers z-index and masking, which I have omitted here because they really need images to make clear.

Overview

Vim 8.2's popup windows allow Vimscript authors and plugin developers to create one or more floating, modal windows that, among other things, can be used for tool tips, notifications/alerts, dialog boxes and transient menus.

Window and Buffer

Each popup is a Vim window and an associated buffer. The window and buffer have a number of unique characteristics including:

Popups cover some or all of vim's regular windows/buffers. They contain no cursor but the cursor in the regular buffer underneath it is visible through the popup window. (For the sake of not losing track of your place, I assume.)
Popups can be stacked on top of each other with stacking order determined by value of each windows' zindex attribute.
The buffer is not 'buflisted', is 'bufhidden' and has a 'buftype' of "popup".
A buffer's initial contents can be specified several ways including by putting the contents in a string or a list of strings. You can also use an existing buffer created with bufadd() (and having a couple other required characteristics).
The buffer has no swapfile or undo capability and is not editable by normal means but the content can be modified post-creation with popup_settext().
The window/buffer look and feel is highly customizable.

Creation

There are six different functions for creating a popup. That's a lot but only one of them is required, popup_create() and the others are "pre-packaged" configurations with particular purposes. Here they are with brief descriptions (from :h popup-functions):

popup_create() - centered in the screen
popup_atcursor() - just above the cursor position, closes on mouse move
popup_beval() - at pos indicated by v:beval_ variables, closes on mouse move
popup_notification() - show a notification for three seconds
popup_dialog() - centered with padding and border
popup_menu() - prompt for selecting an item from a list

Customization

Text appearance can be controlled with syntax highlighting or using text properties (another major new feature in Vim 8.2...like syntax highlighting except it sticks to associated text as it moves in the buffer). There are also a number of attributes or "options" that control certain other look-and-feel characteristics. This is a sampling of those options.

Placement/Size: line, col, pos (e.g. "botright"), maxheight, resize (by mouse), drag (with mouse), minwidth (Note: window height/width is ordinarily based on the width and height of the contained text.)
Appearance: padding, border, borderchars, highlight (a la :hi), title, mask (see example below)
Behavior: time (until close), callback (function), filter (input processing)

Option values can be defined on creation of a popup window or afterwards through the popup_setoptions() function.

Interaction

A popup's lifetime can be controlled several ways including by timer, upon mouse movement or user input or programmatically with popup_close(). In a pinch, there is the nuclear option popup_clear() that will dismiss any and all existing popups.

How a popup processes user input can be controlled with so-called "filters". There are a couple of builtin filters including the "yesno" filter demonstrated in the examples below.

Each popup can have an assigned callback function. When the popup is closed this function is called with the "result". This is most applicable for popups acting as menus. The result, in that case, is usually just the number of the row that the user selected. We'll see this in action, too, in the examples.

Examples

Let's look at examples of some of the features mentioned above.

Notification

For a time-limited notification popup:

call popup_notification("Get the hell out of Dodge!", #{ line: 5, col: 10, highlight: 'WildMenu', } )

As noted above this will show for three seconds then close.

Yes/No Dialog

In this example we'll use the builtin filter popup_filter_yesno. This will wait until the user has clicked a close key (x or <Esc>) or one of Y, y, N, or n. Yes replies cause the callback to be called with result 1. No sends result 0.

func! YayOrNay(id, result)
  if a:result | echo "You said 'yay'" | endif
endfunc

call popup_dialog('[y]ay or [n]ay?', #{ filter: 'popup_filter_yesno', callback: 'YayOrNay'})

If I choose Y or y the message ("You said 'yay'") will show on the status line.

If you specify your own filter function then you can intercept a subset of key presses, handling them in whatever way you choose and pass the rest on to the generic filter, popup_filter_menu(). There's an example at :h popup-examples.

The Base Case

Let's try the base function, popup_create().

call popup_create(poplist, #{ close: 'click' })

Where poplist begins with: [' 1 Vim Launch and App Startup', ' 2 General Use (Editing)', ' 3 Auto Commands', ' 4 Scripting', ' 5 Syntax and Basics', ' 6 Variables', ' 7 Windows', ...etc...]. The idea here is to show what a hierarchical menu (i.e. having depth > 1) looks like.

This is what a popup with all default options looks like (not counting the close: 'click' option so I had an easy way to dismiss it): positioned in the middle of the window, no borders, dimensions dictated by the contained text.

Finishing notes

Okay, hopefully that gives everyone a good feeling for what popups are capable of. There are certainly some features I haven't covered or only mentioned briefly including:

Asynchronous content fill-in

Popup manipulation functions (e.g. popup_move(), popup_setoptions())

User-defined filters

Key mappings

Running a terminal in a popup window

To learn more about those and everything else popup related visit the dedicated 1000+ line chapter in the Vim help docs with :h popups.