[LyX/2.3.x] Fixup b7f6b979: make caret geometry update lazy

[Jean-Marc Lasgouttes]

commit 0e4d7db2be29462fa201c178e6824a95974b04db
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Mon Sep 7 15:45:30 2020 +0200

    Fixup b7f6b979: 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.
    
    (cherry picked from commit c1f8c6c26d5480556db1500160d1df6ce79a724f)
---
 src/frontends/qt4/GuiWorkArea.cpp       |   30 ++++++++++++++++++++++++------
 src/frontends/qt4/GuiWorkArea_Private.h |    4 ++++
 status.23x                              |    2 ++
 3 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/src/frontends/qt4/GuiWorkArea.cpp b/src/frontends/qt4/GuiWorkArea.cpp
index e8cb4c4..2352ecd 100644
--- a/src/frontends/qt4/GuiWorkArea.cpp
+++ b/src/frontends/qt4/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/qt4/GuiWorkArea_Private.h b/src/frontends/qt4/GuiWorkArea_Private.h
index 6342eab..3913016 100644
--- a/src/frontends/qt4/GuiWorkArea_Private.h
+++ b/src/frontends/qt4/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
 
diff --git a/status.23x b/status.23x
index 57da79c..b2e60f9 100644
--- a/status.23x
+++ b/status.23x
@@ -113,6 +113,8 @@ What's new
 
 - Localize some previously unlocalized strings (bug 11954 ).
 
+- Fix bug where cursor blinks at the wrong position after changing layout (bug 11912).
+
 
 * INTERNALS