From d968717940bbad14cabd37ddfca42457adb9e3fb Mon Sep 17 00:00:00 2001
From: Jean-Marc Lasgouttes <lasgouttes@lyx.org>
Date: Mon, 13 Jul 2020 23:29:15 +0200
Subject: [PATCH] Fix paragraph position in TextMetrics::singleParUpdate()

When running redoParagraph, it may happen that the ascent of the first
row changes. Since the ParagraphMetrics position is actually the
baseline of its first row, it may be necessary to update it.

Fixes bug #11601.

(cherry picked from commit 2e8c9cfc832d876ff66e47ae7aadc30fe9328e9d)
---
 src/BufferView.cpp | 11 ++++++++---
 src/update_flags.h |  1 -
 status.23x         |  2 ++
 3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/BufferView.cpp b/src/BufferView.cpp
index c15b7d3b88..e7e69f63e0 100644
--- a/src/BufferView.cpp
+++ b/src/BufferView.cpp
@@ -2754,7 +2754,7 @@ bool BufferView::singleParUpdate()
 	Text & buftext = buffer_.text();
 	pit_type const bottom_pit = d->cursor_.bottom().pit();
 	TextMetrics & tm = textMetrics(&buftext);
-	int old_height = tm.parMetrics(bottom_pit).height();
+	Dimension const old_dim = tm.parMetrics(bottom_pit).dim();
 
 	// make sure inline completion pointer is ok
 	if (d->inlineCompletionPos_.fixIfBroken())
@@ -2765,11 +2765,16 @@ bool BufferView::singleParUpdate()
 	// (if this paragraph contains insets etc., rebreaking will
 	// recursively descend)
 	tm.redoParagraph(bottom_pit);
-	ParagraphMetrics const & pm = tm.parMetrics(bottom_pit);
-	if (pm.height() != old_height)
+	ParagraphMetrics & pm = tm.par_metrics_[bottom_pit];
+	if (pm.height() != old_dim.height()) {
 		// Paragraph height has changed so we cannot proceed to
 		// the singlePar optimisation.
 		return false;
+	}
+	// Since position() points to the baseline of the first row, we
+	// may have to update it. See ticket #11601 for an example where
+	// the height does not change but the ascent does.
+	pm.setPosition(pm.position() - old_dim.ascent() + pm.ascent());
 
 	tm.updatePosCache(bottom_pit);
 
diff --git a/src/update_flags.h b/src/update_flags.h
index a40e88c556..af71c0e78d 100644
--- a/src/update_flags.h
+++ b/src/update_flags.h
@@ -26,7 +26,6 @@ namespace Update {
 		/// Force a full redraw (but no metrics computations)
 		ForceDraw = 4,
 		/// Try to rebreak only the current paragraph metrics.
-		/// (currently ignored!)
 		SinglePar = 8,
 		/// Only the inset decorations need to be redrawn, no text metrics
 		/// update is needed.
diff --git a/status.23x b/status.23x
index 46c3f64fb7..944a0391a2 100644
--- a/status.23x
+++ b/status.23x
@@ -74,6 +74,8 @@ What's new
 
 - Fix the LyX icon in the Wayland dash (bug 11847).
 
+- Fix display bug when typing in a paragraph where row heights change (bug 11601).
+
 
 * INTERNALS