/* ===== Flip Card ===== */
.proof-flip-inner {
    position: relative;
    /* 调整了动画时长和曲线，使扁平翻转更利落 */
    transition: transform 0.5s ease-in-out, height 0.5s ease-in-out;
    transform-style: preserve-3d;
}
.proof-flip-inner.flipped { transform: rotateY(180deg); }
.proof-flip-front { backface-visibility: hidden; }
.proof-flip-back {
    position: absolute; inset: 0;
    backface-visibility: hidden;
    transform: rotateY(180deg);
    background: #fff;
    border-radius: 0.4167vw;
    border: 0.0521vw solid #e5e7eb;
    overflow: hidden;
    display: flex; flex-direction: column;
}

.proof-flip-back .code-header {
    display: flex; align-items: center; justify-content: space-between;
    padding: 0.6vh 0.8vw;
    background: #f3f4f6;
    border-bottom: 0.0926vh solid #e5e7eb;
    flex-shrink: 0;
}
.proof-flip-back .code-header span {
    font-family: 'SF Mono', 'Cascadia Code', 'Fira Code', 'Consolas', 'Courier New', monospace;
    font-size: 12px; color: #6b7280;
}
.proof-flip-back pre {
    margin: 0; padding: 0.8vw;
    font-family: 'SF Mono', 'Cascadia Code', 'Fira Code', 'Consolas', 'Courier New', monospace;
    font-size: 13px; line-height: 1.65;
    color: #374151;
    overflow: auto; flex: 1;
    tab-size: 2;
    white-space: pre;
    counter-reset: line;
    min-height: 0;
}

/* Light syntax theme for Lean source shown on the reverse side. */
.proof-flip-back .code-theme-light.code-comment { color: #6a737d; font-style: italic; }
.proof-flip-back .code-theme-light.code-keyword { color: #d73a49; }
.proof-flip-back .code-theme-light.code-type,
.proof-flip-back .code-theme-light.code-function { color: #6f42c1; }
.proof-flip-back .code-theme-light.code-builtin,
.proof-flip-back .code-theme-light.code-constant,
.proof-flip-back .code-theme-light.code-number { color: #005cc5; }
.proof-flip-back .code-theme-light.code-namespace,
.proof-flip-back .code-theme-light.code-tag { color: #22863a; }
.proof-flip-back .code-theme-light.code-attribute,
.proof-flip-back .code-theme-light.code-heading,
.proof-flip-back .code-theme-light.code-strong { color: #b08800; }
.proof-flip-back .code-theme-light.code-string { color: #032f62; }
.proof-flip-back .code-theme-light.code-operator,
.proof-flip-back .code-theme-light.code-punctuation { color: #24292e; }
.proof-flip-back .code-theme-light.code-emphasis { font-style: italic; }
.proof-flip-back .code-theme-light.code-inserted { color: #22863a; }
.proof-flip-back .code-theme-light.code-deleted,
.proof-flip-back .code-theme-light.code-error { color: #cb2431; }

/* Flip button */
.flip-btn {
    display: inline-flex; align-items: center; gap: 0.2083vw;
    padding: 0.2778vh 0.5208vw;
    font-size: 0.7em; font-weight: 500;
    border: 0 solid;
    background: #fff; color: #6b7280;
    cursor: pointer; transition: all 0.2s;
    vertical-align: middle; user-select: none;
}
.flip-btn:hover { background: #f3f4f6; color: #111827; border-color: #d1d5db; }
.flip-btn i { font-size: 0.6771vw; }