From c1f8c6c26d5480556db1500160d1df6ce79a724f Mon Sep 17 00:00:00 2001
From: Jean-Marc Lasgouttes <lasgouttes@lyx.org>
Date: Mon, 7 Sep 2020 15:45:30 +0200
Subject: [PATCH] Fixup 5202d44e: make caret geometry update lazy

Instead of working around crashes in update of caret geometry, only
request it as needed. The actual computation will take place just
before painting the caret.

It might be that this is overkill and that caret geometry should be
updated unconditionally. One would have to to some timing while idle to
ascertain that.

Fixes bug #11912.
---
 src/frontends/qt/GuiWorkArea.cpp       | 30 ++++++++++++++++++++------
 src/frontends/qt/GuiWorkArea_Private.h |  4 ++++
 2 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/src/frontends/qt/GuiWorkArea.cpp b/src/frontends/qt/GuiWorkArea.cpp
index dd9d1fd5f3..f323fbc3d9 100644
--- a/src/frontends/qt/GuiWorkArea.cpp
+++ b/src/frontends/qt/GuiWorkArea.cpp
@@ -243,7 +243,7 @@ GuiWorkArea::Private::Private(GuiWorkArea * parent)
   caret_visible_(false), need_resize_(false), preedit_lines_(1),
   last_pixel_ratio_(1.0), completer_(new GuiCompleter(p, p)),
   dialog_mode_(false), shell_escape_(false), read_only_(false),
-  clean_(true), externally_modified_(false)
+  clean_(true), externally_modified_(false), needs_caret_geometry_update_(true)
 {
 /* Qt on macOS and Wayland does not respect the
  * Qt::WA_OpaquePaintEvent attribute and resets the widget backing
@@ -487,7 +487,7 @@ void GuiWorkArea::scheduleRedraw(bool update_metrics)
 
 	// update caret position, because otherwise it has to wait until
 	// the blinking interval is over
-	d->updateCaretGeometry();
+	d->resetCaret();
 
 	LYXERR(Debug::WORKAREA, "WorkArea::redraw screen");
 	viewport()->update();
@@ -587,7 +587,7 @@ void GuiWorkArea::Private::resizeBufferView()
 	buffer_view_->resize(p->viewport()->width(), p->viewport()->height());
 	if (caret_in_view)
 		buffer_view_->scrollToCursor();
-	updateCaretGeometry();
+	resetCaret();
 
 	// Update scrollbars which might have changed due different
 	// BufferView dimension. This is especially important when the
@@ -605,6 +605,17 @@ void GuiWorkArea::Private::resizeBufferView()
 }
 
 
+void GuiWorkArea::Private::resetCaret()
+{
+	// Don't start blinking if the cursor isn't on screen.
+	if (!buffer_view_->caretInView())
+		return;
+
+	needs_caret_geometry_update_ = true;
+	caret_visible_ = true;
+}
+
+
 void GuiWorkArea::Private::updateCaretGeometry()
 {
 	// we cannot update geometry if not ready and we do not need to if
@@ -637,18 +648,19 @@ void GuiWorkArea::Private::updateCaretGeometry()
 		&& completer_->completionAvailable()
 		&& !completer_->popupVisible()
 		&& !completer_->inlineVisible();
-	caret_visible_ = true;
 
 	caret_->update(point.x_, point.y_, h, l_shape, isrtl, completable);
+	needs_caret_geometry_update_ = false;
 }
 
 
+
 void GuiWorkArea::Private::showCaret()
 {
 	if (caret_visible_)
 		return;
 
-	updateCaretGeometry();
+	resetCaret();
 	p->viewport()->update();
 }
 
@@ -1359,8 +1371,14 @@ void GuiWorkArea::paintEvent(QPaintEvent * ev)
 	d->paintPreeditText(pain);
 
 	// and the caret
-	if (d->caret_visible_)
+	// FIXME: the code would be a little bit simpler if caret geometry
+	// was updated unconditionally. Some profiling is required to see
+	// how expensive this is (especially when idle).
+	if (d->caret_visible_) {
+		if (d->needs_caret_geometry_update_)
+			d->updateCaretGeometry();
 		d->caret_->draw(pain, d->buffer_view_->horizScrollOffset());
+	}
 
 	d->updateScreen(ev->rect());
 
diff --git a/src/frontends/qt/GuiWorkArea_Private.h b/src/frontends/qt/GuiWorkArea_Private.h
index 6342eaba4a..39130162ad 100644
--- a/src/frontends/qt/GuiWorkArea_Private.h
+++ b/src/frontends/qt/GuiWorkArea_Private.h
@@ -85,6 +85,8 @@ struct GuiWorkArea::Private
 
 	///
 	void dispatch(FuncRequest const & cmd0);
+	/// Make caret visible and signal that its geometry needs to be updated
+	void resetCaret();
 	/// recompute the shape and position of the caret
 	void updateCaretGeometry();
 	/// show the caret if it is not visible
@@ -169,6 +171,8 @@ struct GuiWorkArea::Private
 	bool clean_;
 	///
 	bool externally_modified_;
+	///
+	bool needs_caret_geometry_update_;
 
 }; // GuiWorkArea