Handle metrics of not visible paragraphs

The code is not ready for situations where some paragraphs that are
not visible have metrics available.

In PararagraphMetrics, some methods are added to be able to handle the
fact that paragraphs have or do not have a position.

In TextMetrics, a new method returns the first visible paragraph.

Finally, in BufferView::updateMetrics, the paragraphs' positions are
reset (in the case where everything is not cleared) and some care is
taken to skip the ones that are not relevant.

The assumption outside of this method is that all the paragraphs that
are in the TextMetrics are visible (we are talking about top-level
TextMetrics here). This could be changed (in order to avoid
recomputing paragraph metrics), but the cost is high in terms of
complexity and it is not clear that the gain in terms of performance
would be important.

NOTE: contrary to the code in master which returns npos = -10000, this
code still returns -1 when the position of a paragraph is unknown.

(cherry picked from commit 145af7c2ac)
(cherry picked from commit 05bb851adf)
(cherry picked from commit 8bc3799b35)

src/BufferView.cpp

@@ -3197,6 +3197,7 @@ void BufferView::updateMetrics(bool force)
 		d->text_metrics_.clear();
 	}
 
+	// This should not be moved earlier
 	TextMetrics & tm = textMetrics(&buftext);
 
 	// make sure inline completion pointer is ok
@@ -3212,14 +3213,16 @@ void BufferView::updateMetrics(bool force)
 
 	// Check that the end of the document is not too high
 	int const min_visible = lyxrc.scroll_below_document ? minVisiblePart() : height_;
-	if (tm.last().first == lastpit && tm.last().second->bottom() < min_visible) {
+	if (tm.last().first == lastpit && tm.last().second->hasPosition()
+	     && tm.last().second->bottom() < min_visible) {
 		d->anchor_ypos_ += min_visible - tm.last().second->bottom();
 		LYXERR(Debug::SCROLLING, "Too high, adjusting anchor ypos to " << d->anchor_ypos_);
 		tm.updateMetrics(d->anchor_pit_, d->anchor_ypos_, height_);
 	}
 
 	// Check that the start of the document is not too low
-	if (tm.first().first == 0 && tm.first().second->top() > 0) {
+	if (tm.first().first == 0 && tm.first().second->hasPosition()
+	     && tm.first().second->top() > 0) {
 		d->anchor_ypos_ -= tm.first().second->top();
 		LYXERR(Debug::SCROLLING, "Too low, adjusting anchor ypos to " << d->anchor_ypos_);
 		tm.updateMetrics(d->anchor_pit_, d->anchor_ypos_, height_);
@@ -3232,11 +3235,11 @@ void BufferView::updateMetrics(bool force)
 	 * extra paragraphs are removed
 	 */
 	// Remove paragraphs that are outside of screen
-	while(tm.first().second->bottom() <= 0) {
+	while(!tm.first().second->hasPosition() || tm.first().second->bottom() <= 0) {
 		//LYXERR0("Forget pit: " << tm.first().first);
 		tm.forget(tm.first().first);
 	}
-	while(tm.last().second->top() > height_) {
+	while(!tm.last().second->hasPosition() || tm.last().second->top() > height_) {
 		//LYXERR0("Forget pit: " << tm.first().first);
 		tm.forget(tm.last().first);
 	}

src/ParagraphMetrics.cpp

@@ -40,9 +40,10 @@ using namespace lyx::support;
 
 namespace lyx {
 
+const int pm_npos = -10000;
 
 ParagraphMetrics::ParagraphMetrics(Paragraph const & par) :
-	position_(-1), id_(par.id()), par_(&par)
+	position_(pm_npos), id_(par.id()), par_(&par)
 {}
 
 
@@ -61,7 +62,14 @@ void ParagraphMetrics::reset(Paragraph const & par)
 {
 	par_ = ∥
 	dim_ = Dimension();
-	//position_ = -1;
+	//position_ = pm_npos;
+}
+
+
+int ParagraphMetrics::position() const
+{
+	LASSERT(hasPosition(), return -1);
+	return position_;
 }
 
 
@@ -71,6 +79,18 @@ void ParagraphMetrics::setPosition(int position)
 }
 
 
+void ParagraphMetrics::resetPosition()
+{
+	position_ = pm_npos;
+}
+
+
+bool ParagraphMetrics::hasPosition() const
+{
+	return position_ != pm_npos;
+}
+
+
 Row const & ParagraphMetrics::getRow(pos_type pos, bool boundary) const
 {
 	LBUFERR(!rows().empty());

src/ParagraphMetrics.h

@@ -70,8 +70,12 @@ public:
 	bool hfillExpansion(Row const & row, pos_type pos) const;
 
 	/// The vertical position of the baseline of the first line of the paragraph
-	int position() const { return position_; }
+	int position() const;
 	void setPosition(int position);
+	/// Set position to unknown
+	void resetPosition();
+	/// Return true when the position of the paragraph is known
+	bool hasPosition() const;
 	/// The vertical position of the top of the paragraph
 	int top() const { return position_ - dim_.ascent(); }
 	/// The vertical position of the bottom of the paragraph

src/TextMetrics.cpp

@@ -220,7 +220,10 @@ void TextMetrics::updateMetrics(pit_type const anchor_pit, int const anchor_ypos
                                 int const bv_height)
 {
 	LASSERT(text_->isMainText(), return);
-	pit_type const npit = pit_type(text_->paragraphs().size());
+
+	// Forget existing positions
+	for (auto & pm_pair : par_metrics_)
+		pm_pair.second.resetPosition();
 
 	if (!contains(anchor_pit))
 		// Rebreak anchor paragraph.
@@ -246,6 +249,7 @@ void TextMetrics::updateMetrics(pit_type const anchor_pit, int const anchor_ypos
 	int y2 = anchor_ypos + anchor_pm.descent();
 	// We are now just below the anchor paragraph.
 	pit_type pit2 = anchor_pit + 1;
+	pit_type const npit = pit_type(text_->paragraphs().size());
 	for (; pit2 < npit && y2 < bv_height; ++pit2) {
 		if (!contains(pit2))
 			redoParagraph(pit2);