diff --git a/src/MetricsInfo.cpp b/src/MetricsInfo.cpp
index d663c9a77d..844c1c13f3 100644
--- a/src/MetricsInfo.cpp
+++ b/src/MetricsInfo.cpp
@@ -152,7 +152,8 @@ int MetricsBase::inPixels(Length const & len) const
 
 MetricsInfo::MetricsInfo(BufferView * bv, FontInfo font, int textwidth,
                          MacroContext const & mc, bool vm, bool tight)
-	: base(bv, font, textwidth), macrocontext(mc), vmode(vm), tight_insets(tight)
+	: base(bv, font, textwidth), macrocontext(mc), vmode(vm), tight_insets(tight),
+	  extrawidth(0)
 {}
 
 
diff --git a/src/MetricsInfo.h b/src/MetricsInfo.h
index 176eabfea8..6f1d404822 100644
--- a/src/MetricsInfo.h
+++ b/src/MetricsInfo.h
@@ -109,6 +109,8 @@ public:
 	bool vmode;
 	/// if true, do not expand insets to max width artificially
 	bool tight_insets;
+	/// Extra width required by an inset, in addition to its dimension
+	int extrawidth;
 };
 
 
diff --git a/src/TextMetrics.cpp b/src/TextMetrics.cpp
index 4692918e8e..6968279c23 100644
--- a/src/TextMetrics.cpp
+++ b/src/TextMetrics.cpp
@@ -485,6 +485,7 @@ bool TextMetrics::redoParagraph(pit_type const pit, bool const align_rows)
 	par.setBeginOfBody();
 	Font const bufferfont = buffer.params().getFont();
 	CoordCache::Insets & insetCache = bv_->coordCache().insets();
+	map <Inset const *, int> extrawidths;
 	for (auto const & e : par.insetList()) {
 		// FIXME Doesn't this HAVE to be non-empty?
 		// position already initialized?
@@ -521,6 +522,20 @@ bool TextMetrics::redoParagraph(pit_type const pit, bool const align_rows)
 		MetricsInfo mi(bv_, font.fontInfo(), w, mc, e.pos == 0, tight_);
 		mi.base.outer_font = displayFont(pit, e.pos).fontInfo();
 		e.inset->metrics(mi, dim);
+		/* FIXME: This is a hack. This allows InsetMathHull to state
+		 * that it needs some elbow room beyond its width, in order to
+		 * fit the numbering and/or the left margin (with left
+		 * alignment), which are outside of the inset itself.
+		 *
+		 * To this end, InsetMathHull::metrics() sets a value in
+		 * MetricsInfo::extrawidth and this value is added later to
+		 * the width of the row that contains the inset (when this row
+		 * is tight or shorter than the max allowed width).
+		 *
+		 * See ticket #12320 for details.
+		*/
+		extrawidths[e.inset] = mi.extrawidth;
+
 		if (!insetCache.has(e.inset) || insetCache.dim(e.inset) != dim) {
 			insetCache.add(e.inset, dim);
 			changed = true;
@@ -532,6 +547,12 @@ bool TextMetrics::redoParagraph(pit_type const pit, bool const align_rows)
 	// Split the row in several rows fitting in available width
 	pm.rows() = breakParagraph(bigrow);
 
+	// Add the needed extra width to the rows that contain the insets that request it
+	for (Row & row : pm.rows())
+		for (Row::Element & e : row)
+			if (e.type == Row::INSET && (row.width() < max_width_ || tight_))
+				row.dim().wid += extrawidths[e.inset];
+
 	/* If there is more than one row, expand the text to the full
 	 * allowable width. This setting here is needed for the
 	 * setRowAlignment() below. We do nothing when tight insets are
diff --git a/src/mathed/InsetMathHull.cpp b/src/mathed/InsetMathHull.cpp
index 30ec93a14f..94d293870d 100644
--- a/src/mathed/InsetMathHull.cpp
+++ b/src/mathed/InsetMathHull.cpp
@@ -523,6 +523,9 @@ void InsetMathHull::metrics(MetricsInfo & mi, Dimension & dim) const
 	if (mi.vmode)
 		top_display_margin += theFontMetrics(mi.base.font).maxHeight() + 2;
 
+	int const ind = indent(*mi.base.bv);
+	mi.extrawidth = ind;
+
 	if (previewState(mi.base.bv)) {
 		preview_->metrics(mi, dim);
 		if (previewTooSmall(dim)) {
@@ -554,6 +557,7 @@ void InsetMathHull::metrics(MetricsInfo & mi, Dimension & dim) const
 	if (numberedType()) {
 		BufferParams::MathNumber const math_number = buffer().params().getMathNumber();
 		int extra_offset = 0;
+		int max_nlwid = 0;
 		for (row_type row = 0; row < nrows(); ++row) {
 			rowinfo(row).offset[mi.base.bv] += extra_offset;
 			docstring const nl = nicelabel(row);
@@ -561,7 +565,6 @@ void InsetMathHull::metrics(MetricsInfo & mi, Dimension & dim) const
 				continue;
 			Dimension dimnl;
 			mathed_string_dim(mi.base.font, nl, dimnl);
-			int const ind = indent(*mi.base.bv);
 			int const x = ind ? ind : (mi.base.textwidth - dim.wid) / 2;
 			// for some reason metrics does not trigger at the
 			// same point as draw, and therefore we use >= instead of >
@@ -569,8 +572,10 @@ void InsetMathHull::metrics(MetricsInfo & mi, Dimension & dim) const
 			    || (math_number == BufferParams::RIGHT
 			        && dimnl.wid >= mi.base.textwidth - x - dim.wid)) {
 				extra_offset += dimnl.height();
-			}
+			} else if (dimnl.wid > max_nlwid)
+				max_nlwid = dimnl.wid;
 		}
+		mi.extrawidth += max_nlwid;
 		dim.des += extra_offset;
 	}