Various fixes regarding the video position slider (#8201)

* Prevent default event handling when clicking on the video position slider

This prevents accidental text selection when clicking on the position slider.

* Fix bug when clicking on video position slider before starting the video

* Slightly more aggressive video preloading

- Preload video metadata if the video is loaded in detailed view, as it is
  likely to get played, and metadata is useful for seeking in the video.
- Preload video data if it's fullscreen as it is extremely likely to get
  played right after being put in fullscreen (although those are two steps).
- Preload video data if the user has clicked the position slider, as the video
  will play as soon as the mouse button is released, and video metadata is
  needed to properly seek into the video.

app/javascript/mastodon/features/video/index.js

@@ -158,6 +158,9 @@ export default class Video extends React.PureComponent {
     this.setState({ dragging: true });
     this.video.pause();
     this.handleMouseMove(e);
+
+    e.preventDefault();
+    e.stopPropagation();
   }
 
   handleMouseUp = () => {
@@ -174,8 +177,10 @@ export default class Video extends React.PureComponent {
     const { x } = getPointerPosition(this.seek, e);
     const currentTime = Math.floor(this.video.duration * x);
 
-    this.video.currentTime = currentTime;
+    if (!isNaN(currentTime)) {
-    this.setState({ currentTime });
+      this.video.currentTime = currentTime;
+      this.setState({ currentTime });
+    }
   }, 60);
 
   togglePlay = () => {
@@ -281,6 +286,15 @@ export default class Video extends React.PureComponent {
       playerStyle.height = height;
     }
 
+    let preload;
+    if (startTime || fullscreen || dragging) {
+      preload = 'auto';
+    } else if (detailed) {
+      preload = 'metadata';
+    } else {
+      preload = 'none';
+    }
+
     return (
       <div
         role='menuitem'
@@ -296,7 +310,7 @@ export default class Video extends React.PureComponent {
           ref={this.setVideoRef}
           src={src}
           poster={preview}
-          preload={startTime ? 'auto' : 'none'}
+          preload={preload}
           loop
           role='button'
           tabIndex='0'