fix: stabilize popover positioning and prevent viewport overflow

- Switch popover from position:absolute to position:fixed to avoid
  scroll-offset math and simplify viewport clamping.
- Add max-height + overflow-y:auto so the popover scrolls internally
  instead of overflowing the viewport.
- Remove the reposition call after picker opens — this was causing
  the popover to flash and jump when the picker changed its height.
- Improve edge clamping: if neither above nor below fits, clamp to
  viewport bottom rather than flipping unpredictably.
- Remove the CSS arrow (::before) which was mispositioned when the
  popover appeared above the badge or near edges.

Co-authored-by: Cursor <cursoragent@cursor.com>

content/tweaks/color-popover.js

@@ -161,7 +161,6 @@
       row.hex.classList.remove("rt-input-error");
       updatePreview();
     });
-    if (currentAnchor) positionPopover(currentAnchor);
   }
 
   function closePicker() {
@@ -238,21 +237,29 @@
   function positionPopover(anchorEl) {
     var rect = anchorEl.getBoundingClientRect();
     var gap = 6;
+    var pad = 10;
 
     popoverEl.style.visibility = "hidden";
     popoverEl.classList.add("rt-popover-visible");
 
     var popRect = popoverEl.getBoundingClientRect();
-    var top = rect.bottom + window.scrollY + gap;
-    var left = rect.left + window.scrollX;
 
-    if (rect.bottom + gap + popRect.height > window.innerHeight) {
-      top = rect.top + window.scrollY - popRect.height - gap;
+    var top = rect.bottom + gap;
+    var left = rect.left;
+
+    if (top + popRect.height > window.innerHeight - pad) {
+      var aboveTop = rect.top - popRect.height - gap;
+      if (aboveTop >= pad) {
+        top = aboveTop;
+      } else {
+        top = Math.max(pad, window.innerHeight - popRect.height - pad);
+      }
     }
 
-    var maxLeft = document.documentElement.scrollWidth - popRect.width - 8;
-    if (left > maxLeft) left = maxLeft;
-    if (left < 8) left = 8;
+    if (left + popRect.width > window.innerWidth - pad) {
+      left = window.innerWidth - popRect.width - pad;
+    }
+    if (left < pad) left = pad;
 
     popoverEl.style.top = top + "px";
     popoverEl.style.left = left + "px";