Fix drag issue

These few lines of code are going to plague this project.
Transiently, this solution doesn't seem fast enough and
will "forget" that the mouse button is being held during
a drag.
I'm using mouseenter instead of mouseover because it
should trigger less often---this ought to be more efficient(?).

src/components/source_plate.rs

@@ -68,7 +68,10 @@ fn SourcePlateCell(cx: Scope<PlateCellProps>, i: u8, j: u8, color: Option<String
                 selection_state.write().m_end = None;
                 selection_state.write().m_stat = true;
             },
-            onmouseover: move |_| {
+            onmouseenter: move |me: MouseEvent| {
+                if me.data.held_buttons().is_empty() {
+                    selection_state.write().m_stat = false;
+                }
                 if selection_state.read().m_stat {
                     selection_state.write().m_end = Some((*i,*j))
                 }