Fix horizontal scrolling in full-width collapsable insets

While a one paragraph large collapsable inset (containing for example a tabular) could be very wide and trigger horizontal scrolling, the code that makes collapsable insets wide when they contain several paragraphs would actually make them narrower in this case.

Typical example is a wide tabular and a caption in a table float, where horizontal scrolling would not trigger.

(cherry picked from commit a879bc2575)

src/TextMetrics.cpp

@@ -448,7 +448,7 @@ bool TextMetrics::redoParagraph(pit_type const pit)
 		setRowHeight(row, pit);
 		row.setChanged(false);
 		if (row_index || row.endpos() < par.size()
-			|| (row.right_boundary() && par.inInset().lyxCode() != CELL_CODE))
+		    || (row.right_boundary() && par.inInset().lyxCode() != CELL_CODE)) {
 			/* If there is more than one row or the row has been
 			 * broken by a display inset or a newline, expand the text
 			 * to the full allowable width. This setting here is
@@ -458,7 +458,9 @@ bool TextMetrics::redoParagraph(pit_type const pit)
 			 * that, and it triggers when using a caption in a
 			 * longtable (see bugs #9945 and #9757).
 			 */
+			if (dim_.wid < max_width_)
 				dim_.wid = max_width_;
+		}
 		int const max_row_width = max(dim_.wid, row.width());
 		computeRowMetrics(pit, row, max_row_width);
 		first = row.endpos();

status.22x

@@ -51,6 +51,9 @@ What's new
 
 - Fix on screen narrow box when using \width as box width unit (bug 10048).
 
+- Fix horizontal scrolling feature when inside a collapsable inset
+  with several paragraphs.
+
 - Raise GuiView on buffer switch to bring it topmost in multi-window mode.