fix(ui): center split pane resize handle (#11506)

Co-authored-by: Hiroshi Ogawa <4232207+hi-ogawa@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: hi-ogawa

packages/ui/client/styles/main.css

@@ -10,7 +10,7 @@ body {
   --color-text-dark: #ddd;
   --color-text: var(--color-text-light);
   --background-color: #e4e4e4;
-  --splitpane-handle-size: 5px;
+  --splitpane-handle-size: 2px;
 }
 
 html.dark {
@@ -122,20 +122,18 @@ html.dark {
   z-index: 1;
 }
 
-.splitpanes__splitter:hover:before {
+.splitpanes__splitter:hover:before,
+.splitpanes--dragging > .splitpanes__splitter:before {
   opacity: 1;
 }
 
 .splitpanes--vertical > .splitpanes__splitter:before {
-  /* make vertical scroll usable */
-  left: 0;
-  right: calc(var(--splitpane-handle-size) * -1);
+  inset-inline: calc(var(--splitpane-handle-size) * -1);
   height: 100%;
 }
 
 .splitpanes--horizontal > .splitpanes__splitter:before {
-  top: 0;
-  bottom: calc(var(--splitpane-handle-size) * -1);
+  inset-block: calc(var(--splitpane-handle-size) * -1);
   width: 100%;
 }