Settle the theme and header before the first paint

2 files changed, 38 insertions, 14 deletions

diff --git a/assets/cgit.css b/assets/cgit.css
index 878e96c..0f0c898 100644
--- a/assets/cgit.css
+++ b/assets/cgit.css
@@ -3,7 +3,7 @@
  *
  * Colours are driven by CSS custom properties via light-dark(), so the
  * whole theme follows the reader's system preference and can be flipped
- * with the header toggle (which sets data-theme on div#cgit). All rules
+ * with the header toggle (which sets data-theme on <html>). All rules
  * stay scoped under div#cgit so cgit can be embedded in another page.
  */
 
@@ -103,9 +103,11 @@ div#cgit {
 	background: var(--bg);
 }
 
-/* Explicit theme choices made through the header toggle. */
-div#cgit[data-theme="light"] { color-scheme: light; }
-div#cgit[data-theme="dark"]  { color-scheme: dark; }
+/* Explicit theme choices made through the header toggle. The marker is on
+ * <html> rather than div#cgit so the script can set it before the body is
+ * parsed, which is what keeps the first paint from showing the other theme. */
+html[data-theme="light"] div#cgit { color-scheme: light; }
+html[data-theme="dark"]  div#cgit { color-scheme: dark; }
 
 /* On a standalone cgit page, paint the page background to match the app
  * and drop the default body margin so the interface is part of the page
@@ -257,10 +259,20 @@ div#cgit #header .theme-toggle svg {
 /* Reserve the logo column. The image carries no width attribute, since the
  * logo is configurable and its size is not known here, so without this the
  * column collapses to nothing and the masthead jumps sideways on load. */
+/* The toggle starts hidden and is revealed once the script has run, so its
+ * column is reserved up front to keep the header from jumping. Only when
+ * scripting is on, otherwise the space would sit empty forever. */
+html.cgit-js div#cgit #header {
+	grid-template-columns: auto 1fr auto var(--control-h);
+}
+
+/* Reserve the logo's box so the header does not grow when the image
+ * arrives. The height matches the shipped 192x128 logo capped to 96px. */
 div#cgit #header .logo {
 	grid-area: logo;
 	align-self: start;
 	min-width: 96px;
+	min-height: 64px;
 }
 
 div#cgit #header .logo img {
@@ -1406,6 +1418,7 @@ div#cgit table.ssdiff td.space div {
 	}
 	div#cgit #header .logo {
 		min-width: 56px;
+		min-height: 37px;
 	}
 	div#cgit #header .logo img {
 		max-width: 56px;
diff --git a/assets/cgit.js b/assets/cgit.js
index 6fe53ba..f3fedc1 100644
--- a/assets/cgit.js
+++ b/assets/cgit.js
@@ -160,36 +160,47 @@ function persist(value) {
 	try { localStorage.setItem(KEY, value); } catch (e) { }
 }
 
-function apply(theme, root, btn) {
+function apply(theme, btn) {
+	var html = document.documentElement;
 	if (theme === "auto")
-		root.removeAttribute("data-theme");
+		html.removeAttribute("data-theme");
 	else
-		root.setAttribute("data-theme", theme);
+		html.setAttribute("data-theme", theme);
 	/* Keep the standalone page background in sync with the choice. */
-	document.documentElement.style.colorScheme = (theme === "auto") ? "" : theme;
+	html.style.colorScheme = (theme === "auto") ? "" : theme;
 	if (btn) {
 		btn.innerHTML = ICONS[theme];
 		btn.title = "Colour theme: " + theme;
 	}
 }
 
+var theme = saved();
+if (ORDER.indexOf(theme) < 0)
+	theme = "auto";
+
+/* Settled here rather than on DOMContentLoaded. This file is fetched from
+ * <head> and runs before the body is parsed, so the choice lands on <html>
+ * ahead of the first paint and the page never shows the other theme first.
+ * The marker sits on <html> for the same reason, since div#cgit does not
+ * exist yet. cgit-js reserves the toggle's column so revealing the button
+ * below does not shift the header. */
+apply(theme, null);
+document.documentElement.classList.add("cgit-js");
+
 document.addEventListener("DOMContentLoaded", function () {
 	var root = document.getElementById("cgit");
 	if (!root)
 		return;
 	var btn = root.querySelector(".theme-toggle");
-	var theme = saved();
-	if (ORDER.indexOf(theme) < 0)
-		theme = "auto";
-
-	apply(theme, root, btn);
 	if (!btn)
 		return;
+
+	apply(theme, btn);
 	btn.hidden = false;
 	btn.addEventListener("click", function () {
 		theme = ORDER[(ORDER.indexOf(theme) + 1) % ORDER.length];
 		persist(theme);
-		apply(theme, root, btn);
+		apply(theme, btn);
 	});
 }, false);