Place tagging whatsits in an hbox

CHANGELOG.md

@@ -16,6 +16,8 @@ Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to
 
 ### Fixed
 
+- Handling of whatsits at start of slide box (see issue
+  [\#217](https://github.com/josephwright/ltx-talk/issues/217))
 - Behavior of `\temporal` (see issue
   [\#224](https://github.com/josephwright/ltx-talk/issues/224))
 

ltx-talk-frame.dtx

@@ -113,7 +113,7 @@
 \property_new:nnnn { slides } { now } { 1 } { \int_use:N \g_@@_slide_int }
 %    \end{macrocode}
 %
-% \begin{macro}{\@@_slide:nn}
+% \begin{macro}{\@@_slide:nn, \@@_slide__aux:nn}
 % \begin{macro}{\@@_slide_aux:n}
 %  Each slide is parsed inside simple set up, the only complexity being if we
 %  are handling fragile frames. There, all \cs{obeyedline} in the grabbed
@@ -143,38 +143,53 @@
       \int_gzero:N \g_@@_slide_int
       \RenewCommandCopy \frame \@@_latexe_frame:n
       \bool_do_while:Nn \g_@@_slide_continue_bool
-        {
-          \int_gincr:N \g_@@_slide_int
-          \bool_gset_false:N \g_@@_slide_continue_bool
-          \@@_if_overlay:nT {#1}
-            {
-              \@@_slide_begin:
-              \bool_set_eq:NN \l_@@_slide_continue_bool
-                \g_@@_slide_continue_bool
-              \@@_if_overlay:VTF \l_@@_frame_tagging_str
-                {
-                  \bool_gset_eq:NN \g_@@_slide_continue_bool
-                    \l_@@_slide_continue_bool
-                  \@@_frame_tag:n
-                }
-                {
-                  \bool_gset_eq:NN \g_@@_slide_continue_bool
-                    \l_@@_slide_continue_bool
-                  \@@_frame_notag:n
-                }
-                  {
-                    \bool_if:NTF \l_@@_frame_verb_bool
-                      { \@@_slide_aux:n }
-                      { \use:n }
-                        {#2}
-                  }
-               \@@_slide_end:
-            }
-        }
+        { \@@_slide_aux:nn {#1} {#2} }
       \property_record:ee { frame . \int_use:N \g_@@_frame_int }
         { slides }
     \group_end:
   }
+%    \end{macrocode}
+%.  At the start of each slide, we are in vertical mode. That is switched back
+%.  to horizontal mode before we deal with any tagging whatsits, then back to
+%.  vertical mode once this is sorted.
+%    \begin{macrocode}
+\cs_new_protected:Npn \@@_slide_aux:nn #1#2
+  {
+    \int_gincr:N \g_@@_slide_int
+    \bool_gset_false:N \g_@@_slide_continue_bool
+    \@@_if_overlay:nT {#1}
+      {
+        \@@_slide_begin:
+        \hbox:n
+          {
+            \tl_gclear:N \g_@@_onslide_tl
+            \bool_set_eq:NN \l_@@_slide_continue_bool
+              \g_@@_slide_continue_bool
+            \@@_if_overlay:VTF \l_@@_frame_tagging_str
+              {
+                \bool_gset_eq:NN \g_@@_slide_continue_bool
+                  \l_@@_slide_continue_bool
+                \@@_frame_tag:n
+              }
+              {
+                \bool_gset_eq:NN \g_@@_slide_continue_bool
+                  \l_@@_slide_continue_bool
+                \@@_frame_notag:n
+              }
+                {
+                  \vbox:n
+                    {
+                      \bool_if:NTF \l_@@_frame_verb_bool
+                        { \@@_slide_aux:n }
+                        { \use:n }
+                          {#2}
+                    }
+                }
+            \tl_use:N \g_@@_onslide_tl
+          }
+        \@@_slide_end:
+      }
+  }
 \cs_new_protected:Npn \@@_slide_aux:n #1
   {
     \group_begin:
@@ -208,6 +223,9 @@
 % \end{variable}
 %
 % \begin{macro}{\@@_slide_begin:, \@@_slide_end:}
+%   Start and end the box holding all of the slide: we are in vertical mode
+%   here, and need to swap back to horizontal mode above to deal with
+%   whatsits.
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_slide_begin:
   {
@@ -217,11 +235,9 @@
     \box_gclear:N \g_@@_footnote_box
     \@@_cnt_save:
     \vbox_set:Nw \l_@@_slide_box
-      \tl_gclear:N \g_@@_onslide_tl
   }
 \cs_new_protected:Npn \@@_slide_end:
   {
-      \tl_use:N \g_@@_onslide_tl
     \vbox_set_end:
     \bool_if:NT \g_@@_slide_continue_bool
       { \@@_cnt_restore: }