Function popup_outer_size 

pub fn popup_outer_size(
    buffer_width: u16,
    buffer_height: u16,
    line_count: u16,
    placement: PopupPlacement,
) -> (u16, u16)

Outer size (border-inclusive) of a help / hover popup overlay.

Centred popups are reading surfaces (:help, :options, :describe-*, :apropos, :customize); they want enough room to lay out paragraphs + tables comfortably without covering the whole screen. The buffer below stays partly visible so the user keeps spatial context. Caps:

• Width: min(buffer_width - 4, 120), floor 30. The 120-cell ceiling preserves a comfortable line length for reading markdown – wider lines are harder to scan, so even on a 200-cell terminal the popup stops at 120.
• Height: buffer_height * 3 / 4, floor 5, no upper ceiling. Three-quarters leaves a strip of the underlying buffer visible at top and bottom while giving a large help doc all the vertical room the screen allows; anything longer than the box scrolls within the popup.

Cursor-anchored popups are tooltips (hover, signature help); they sit adjacent to the cursor and want to not dominate the screen. Caps stay tight: width 30..=80, height 5..=20.

line_count is the popup’s content row count (excluding borders). The returned (width, height) is border-inclusive (height = inner + 2); subtract 2 to get the inner viewport for motion / scroll.