Opacity — the game behind the panel

Popups, dialogs, sheets, cards, HUD panels, alerts and snackbars draw their face at 80% opacity. The fight carries on behind a confirm dialog; the map shows under an inventory sheet. In a game this is not decoration — it is what keeps the player oriented. A fully solid panel erases where they were standing the moment a window opens.

Five layers, most specific first

With none of them set the value is 100, so nothing about an older theme changes.

# One window only — a confirm dialog that must not hide the fight behind it
surface.alpha = 0.6
sheet.alpha = 0.7
dialogs.alpha = 0.9                          # @export fields are ratios too
drawer.alpha = 0.7
GoPopover.open(slot, body, {"alpha": 0.9})
var glass := GoStyle.card(Color.TRANSPARENT, -1.0, -1.0, -1.0, 0.5)

# Per kind — keep the HUD nearly solid, because its text sits straight over the world
GoUi.config.container_alpha_overrides = {
    GoTheme.BOX_PANEL: 0.7,                   # dialogs and sheets can breathe
    GoTheme.BOX_HUD: 0.95,
}
GoUi.refresh()                               # redraws widgets that are already open

# The whole project — a busy world wants its panels back to solid
GoUi.config.container_alpha = 1.0
GoUi.refresh()

GoUi.surface_alpha(GoTheme.BOX_PANEL)        # the resolved ratio, e.g. 0.8

What follows the value, and what does not

A button whose state is washed out no longer says what it is, and a badge exists to make two characters legible — neither is a container, so neither follows the value.

Three details that come from real screens

• The value multiplies, it does not replace. The default HUD face is already 0.92 alpha, so hud_alpha 80 lands at 0.736 — a theme that deliberately made a face translucent keeps that decision. The flip side: apply it twice and the panel fades twice, which is why the alpha is multiplied in exactly one place (GoSkin.surface_box()) and every other face function forwards the argument instead of fading its own result.
• A colour you passed is a colour you get. GoStyle.plate(…, fill) keeps an explicit fill exactly as given: Color(ink, 0.14) already states its alpha, and multiplying panel opacity into it would cut the caller's intent twice. GoStyle.floating(…, opaque = true) ignores the value for the same kind of reason — filling the face solid is that flag's whole purpose.
• Panels gohud did not build get the same rule. GoStyle.fade_panel(node) — after the node is in the tree, so it reads the theme it inherits. It is idempotent: the original face is recorded and every call recomputes from that, so repeated redraws never stack. After swapping themes, GoStyle.forget_face(node) drops that memory so the next call picks up the new face.

Custom faces fade too — and that is the part that silently breaks

The cut-corner panel (GoStyleBoxCut) and the forged frame (GoStyleBoxMedieval) are not StyleBoxFlat; they draw themselves. Put the opacity only in the flat branch and these themes quietly ignore it while every value-based test still passes. Both fade their background and keep glow, rivets, corner engraving and bevels at full strength — and the medieval face's grain and bevel were always proportional to its background alpha, so they thin out along with the panel.

Which is the wider point: bg_color.a == 0.8 is a number. The world shows through and the text is still legible is the requirement. gohud checks the first in its headless suite and the second by drawing panels over diagonal stripes on a virtual monitor (tests/gohud_alpha_shot.gd) — on the flat, the cut and the forged face, in both the dark and the light theme.