Fix drawing of the left-most AUI notebook tab under Windows 10

Apparently, in some situations the left edge of the first tab is not
drawn when it's not selected under Windows 10. While it doesn't seem to
affect all the systems (and couldn't be reproduced here), drawing the
left edge explicitly shouldn't do any harm and should fix the problem
for the systems that are affected by it, so just do it always.

Closes #17714.

src/aui/tabartmsw.cpp

@@ -232,6 +232,22 @@ void wxAuiMSWTabArt::DrawTab(wxDC& dc,
         tabState,
         &tabR, NULL);
 
+    // Apparently, in at least some Windows 10 installations the call above
+    // does not draw the left edge of the first tab and it needs to be drawn
+    // separately, or it wouldn't be drawn at all.
+    if ( tabX == GetIndentSize() )
+    {
+        te->DrawThemeBackground
+            (
+                hTabTheme,
+                GetHdcOf(dc.GetTempHDC()),
+                TABP_TABITEMLEFTEDGE,
+                tabState,
+                &tabR,
+                NULL
+            );
+    }
+
     wxRect textRect = tabRect;
     if ( !page.active )
         textRect.Offset(0, wnd->FromDIP(1));