add stable keyboard scrolling to vim floating preview popups

[coriocactus]

Vim floating previews for hover and ALEDetail can't be scrolled from the keyboard. When the text is longer than the popup, the only way to read the rest is the mouse, and moved: 'any' closes the popup on any cursor motion.

This adds a popup filter, used in Normal mode only:

• When a preview has more text than fits, Vim's own scroll keys scroll the preview instead of the buffer: CTRL-E/CTRL-Y by a line, CTRL-D/CTRL-U by half a page, CTRL-F/CTRL-B by a page.
• <Esc> closes the preview.
• Other keys pass through. If all the text already fits, the scroll keys pass through too, so short popups behave as they do now.

How it works:

• Each key runs as a counted CTRL-D or CTRL-U inside the popup with win_execute(). Vim stops these at the start and end of the text, wrapped or not, so the popup never scrolls into blank space and its height never changes. ALE doesn't do any scroll math of its own. The filter redraws after each scroll: without that, Vim 8.2.4843 scrolls wrapped text one line too far when keys arrive faster than redraws (found by the Vim 8.2 Windows CI job).
• The mouse wheel is left alone. Vim already scrolls a popup with the wheel when the pointer is over it.
• Before patch 9.2.0419, Vim works out a popup's width from the lines currently in view, so the width jumps around while scrolling. On those versions ALE sets minwidth to the widest line, unless the user has set minwidth or maxwidth. The check is gated on has('patch-9.2.0419'), so it can be deleted when ALE's minimum Vim version passes that patch.
• The filter is script-local, so this adds no new public API. Setting filter in g:ale_floating_preview_popup_opts still replaces it.

Neovim floating windows are unchanged.

Tests cover line, half-page and full-page scrolling with stops at both ends, reaching the end of wrapped text, keys passing through when nothing needs scrolling, other keys passing through, <Esc>, the width fix, and user width options being kept. One test sends keys through feedkeys() to check that Vim really delivers them to the filter in Normal mode. They pass on Vim 8.2 and 9.2. On Neovim they are skipped, because it has no popupwin.

[Copilot]

Pull request overview

Adds keyboard (and mouse wheel) scrolling support for Vim popupwin-based floating preview windows in ALE, plus sizing stabilization to prevent resize jitter while scrolling. NeoVim floating previews are unchanged.

Changes:

• Introduces a Vim popup filter (normal-mode filtermode) to handle <C-n>/<C-p>, <C-d>/<C-u>, mouse wheel, and <Esc> for floating preview popups.
• Adds “managed” width/height locking for Vim popups when users haven’t provided explicit size limits.
• Adds a new Vader test suite covering scrolling behavior, boundary clamping, closing on escape, passthrough keys, and size locking; updates docs accordingly.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
autoload/ale/floating_preview.vim	Implements popup filter, scroll/clamp logic, and managed size locking/reset for Vim popup previews.
doc/ale.txt	Documents the new scrolling keys and sizing behavior for Vim popup previews and how to override the filter.
test/test_floating_preview_popupwin.vader	Adds coverage for popup filter behavior, scroll boundaries, escape close, and size stability during scrolling.

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[w0rp]

I'll paste this in every open PR: We finally managed to migrate CI checks for Windows from AppVeyor to GitHub Actions, and we've updated the minimum supported Vim version to 8.2. (Thought Vim 8.0 might happen to work for some.) In order to get checks to run again for a PR you'll need to rebase or merge.

[rymdbar]

How to try out this pull-request:

0. Ensure vim, ale (doh!), cargo and rust-analyzer are installed.
1. Configure .vimrc to contain let g:ale_floating_preview = 1 and let g:ale_linters.rust = [ 'rustc' ].
2. Create a rust project.

mkdir pr5114
cd pr5114/
cargo init

3. Update src/main.rs to contain:

fn main() {
    let x = 42;
    let y: String = x;
}

4. Resize vim to be only 5-6 lines tall. Run :ALEDetail and observe the problem as described.

5. Checkout pull-request branch.

6. Observe how the issue is solved.