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/styles/badges.css

@@ -86,14 +86,16 @@ html.rt-enabled .thing.link .flat-list li:has(> .comments) {
 
 /* --- Color popover --- */
 .rt-color-popover {
-  position: absolute;
+  position: fixed;
   z-index: 10000;
   background: #fff;
   border: 1px solid #ccc;
   border-radius: 6px;
-  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.15);
+  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.18);
   padding: 12px;
   width: 260px;
+  max-height: calc(100vh - 20px);
+  overflow-y: auto;
   font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
   font-size: 13px;
   color: #1a1a1a;
@@ -104,20 +106,6 @@ html.rt-enabled .thing.link .flat-list li:has(> .comments) {
   display: block;
 }
 
-/* Popover arrow */
-.rt-color-popover::before {
-  content: "";
-  position: absolute;
-  top: -6px;
-  left: 16px;
-  width: 10px;
-  height: 10px;
-  background: #fff;
-  border-left: 1px solid #ccc;
-  border-top: 1px solid #ccc;
-  transform: rotate(45deg);
-}
-
 .rt-popover-header {
   display: flex;
   justify-content: space-between;
@@ -275,12 +263,6 @@ html.rt-enabled .thing.link .flat-list li:has(> .comments) {
   color: #e0e0e0;
 }
 
-.rt-color-popover.rt-dark::before {
-  background: #2a2a2a;
-  border-left-color: #555;
-  border-top-color: #555;
-}
-
 .rt-color-popover.rt-dark .rt-popover-header {
   border-bottom-color: #444;
 }