add stable keyboard scrolling to vim floating preview popups

[coriocactus]

vim floating preview windows for hover and detail output had no keyboard scrolling, so long content required the mouse to read

this adds a popup filter for Vim popupwin previews that handles / for line scrolling, / for half-page scrolling, mouse wheel events, and to dismiss; unhandled keys pass through normally and moved: 'any' still closes the popup on cursor movement; to prevent resize jitter while scrolling, ALE locks popup width to the maximum content display width and height to the rendered height when users have not set size limits themselves, and disables resize while those managed dimensions are active; scroll clamping at both boundaries prevents the viewport from moving into blank space, and the bottom boundary accounts for line wrapping as well as wrap: 0; users can override the filter or supply explicit sizing via g:ale_floating_preview_popup_opts, and neovim floating windows are unchanged

the filter runs in filtermode: 'n' so it does not interfere with insert-mode completion keys; mapping is left at the default so user normal-mode mappings are respected, which means custom mappings on etc will take precedence over popup scrolling; the size-locking cleanup uses unlet! defensively so future refactors cannot hit a missing-key error on managed_size

[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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[Copilot]

The new key references are formatted as Vim help links (e.g. |<C-n>|, |<Esc>|), but these aren’t standard help tags and will render as broken links. In this help file, key help tags are typically referenced as |CTRL-N|, |CTRL-P|, etc. Consider switching to the existing tag style (or removing |...| markup and just using <C-n>/<Esc> as plain text).

Suggested change

	keyboard and dismissed with |<Esc>|. The following keys are supported:
	|<C-n>| - Scroll down one line
	|<C-p>| - Scroll up one line
	|<C-d>| - Scroll down half a page
	|<C-u>| - Scroll up half a page
	|<Esc>| - Close the popup
	keyboard and dismissed with <Esc>. The following keys are supported:
	<C-n> - Scroll down one line
	<C-p> - Scroll up one line
	<C-d> - Scroll down half a page
	<C-u> - Scroll up half a page
	<Esc> - Close the popup