using el.videoHeight here does help

The whole thing appears to be a hack, though;
we should really be using the image dimensions
from the file info.

builds/4chan-X.user.js

@@ -7719,7 +7719,9 @@
         $.after(position, el);
       }
       return $.asap((function() {
-        return isVideo || el.naturalHeight;
+        if (isVideo) {
+          return el.videoHeight || el.naturalHeight;
+        }
       }), function() {
         return ImageExpand.completeExpand(post);
       });
@@ -7967,7 +7969,9 @@
         latestEvent: e,
         endEvents: 'mouseout click',
         asapTest: function() {
-          return isVideo || el.naturalHeight;
+          if (isVideo) {
+            return el.videoHeight || el.naturalHeight;
+          }
         },
         noRemove: true,
         cb: function() {

builds/crx/script.js

@@ -7738,7 +7738,9 @@
         $.after(position, el);
       }
       return $.asap((function() {
-        return isVideo || el.naturalHeight;
+        if (isVideo) {
+          return el.videoHeight || el.naturalHeight;
+        }
       }), function() {
         return ImageExpand.completeExpand(post);
       });
@@ -7975,7 +7977,9 @@
         latestEvent: e,
         endEvents: 'mouseout click',
         asapTest: function() {
-          return isVideo || el.naturalHeight;
+          if (isVideo) {
+            return el.videoHeight || el.naturalHeight;
+          }
         },
         noRemove: true,
         cb: function() {

src/Images/ImageExpand.coffee

@@ -117,7 +117,7 @@ ImageExpand =
       el.src = src or post.file.URL
     position = if isVideo and Conf['Show Controls'] then thumb.parentNode else thumb
     $.after position, el unless el is position.nextSibling
-    $.asap (-> isVideo or el.naturalHeight), ->
+    $.asap (-> if isVideo then el.videoHeight or el.naturalHeight), ->
       ImageExpand.completeExpand post
 
   completeExpand: (post) ->

src/Images/ImageHover.coffee

@@ -37,7 +37,7 @@ ImageHover =
       el: el
       latestEvent: e
       endEvents: 'mouseout click'
-      asapTest: -> (isVideo or el.naturalHeight)
+      asapTest: -> (if isVideo then el.videoHeight or el.naturalHeight)
       noRemove: true
       cb: ->
         if isVideo