Fix 5438: Keep floating player in view area

## Issue
5438: Keep floating player in view area

## Changes
- Restore correct z-index
    -  The bug came as a side effect of cbac2174. Redo cbac2174 to cover both close button and floating player.

- Remove wobbling when dragging to bottom-right.
    -  The extra gap introduced is fighting against react-draggable. If gaps are needed, handle it entire ourselves or do it somewhere in react-draggable; neither is worth the effort, so the gap was removed.

ui/component/fileRenderFloating/view.jsx

@@ -89,17 +89,16 @@ export default function FileRenderFloating(props: Props) {
   }
 
   function clampToScreen(pos) {
-    const GAP_PX = 10;
     const ESTIMATED_SCROLL_BAR_PX = 50;
     const FLOATING_PLAYER_CLASS = 'content__viewer--floating';
     const fpPlayerElem = document.querySelector(`.${FLOATING_PLAYER_CLASS}`);
 
     if (fpPlayerElem) {
       if (pos.x + fpPlayerElem.getBoundingClientRect().width > getScreenWidth() - ESTIMATED_SCROLL_BAR_PX) {
-        pos.x = getScreenWidth() - fpPlayerElem.getBoundingClientRect().width - ESTIMATED_SCROLL_BAR_PX - GAP_PX;
+        pos.x = getScreenWidth() - fpPlayerElem.getBoundingClientRect().width - ESTIMATED_SCROLL_BAR_PX;
       }
       if (pos.y + fpPlayerElem.getBoundingClientRect().height > getScreenHeight()) {
-        pos.y = getScreenHeight() - fpPlayerElem.getBoundingClientRect().height - GAP_PX * 2;
+        pos.y = getScreenHeight() - fpPlayerElem.getBoundingClientRect().height;
       }
     }
   }

ui/scss/component/_content.scss

@@ -15,6 +15,7 @@
   height: calc(var(--floating-viewer-height) + var(--floating-viewer-info-height));
   overflow: hidden;
   left: calc(var(--spacing-l) + var(--spacing-s));
+  z-index: 9999;
 
   &:hover {
     .content__floating-close {