Fix: 'Cache' top and bottom lines of textfile viewer to avoid overdraw. (#9131)

* Fix: 'Cache' top and bottom lines of textfile viewer to avoid overdraw.

The text file viewer calculated the number of lines required to set the scrollbar, but did not retain this information, so this was recalculated on every draw operation. This includes overdrawing text outside the bounds of the current scroll position.

With this change the top and bottom lines for each line of text are remembered, and reflowing is avoided where possible. Text outside the current scroll bounds is not drawn.
Additionally the scroll interval is now based on text lines instead of pixel lines, which increases the text capacity depending on the font size.

* Fix: Limit text viewer to showing 64k lines.

Text files with more than 64k wrapped lines would exceed the scrollbar capacity and cause an assert. This is harder to reach now that the scrollbar counts lines instead of pixels.

1 files changed, 56 insertions, 25 deletions

diff --git a/src/textfile_gui.cpp b/src/textfile_gui.cpp
index 075898fd7..ac8da5c2e 100644
--- a/src/textfile_gui.cpp
+++ b/src/textfile_gui.cpp
@@ -67,7 +67,6 @@ TextfileWindow::TextfileWindow(TextfileType file_type) : Window(&_textfile_desc)
 	this->GetWidget<NWidgetCore>(WID_TF_CAPTION)->SetDataTip(STR_TEXTFILE_README_CAPTION + file_type, STR_TOOLTIP_WINDOW_TITLE_DRAG_THIS);
 
 	this->hscroll->SetStepSize(10); // Speed up horizontal scrollbar
-	this->vscroll->SetStepSize(FONT_HEIGHT_MONO);
 }
 
 /* virtual */ TextfileWindow::~TextfileWindow()
@@ -79,23 +78,38 @@ TextfileWindow::TextfileWindow(TextfileType file_type) : Window(&_textfile_desc)
  * Get the total height of the content displayed in this window, if wrapping is disabled.
  * @return the height in pixels
  */
-uint TextfileWindow::GetContentHeight()
+uint TextfileWindow::ReflowContent()
 {
-	int max_width = this->GetWidget<NWidgetCore>(WID_TF_BACKGROUND)->current_x - WD_FRAMETEXT_LEFT - WD_FRAMERECT_RIGHT;
-
 	uint height = 0;
-	for (uint i = 0; i < this->lines.size(); i++) {
-		height += GetStringHeight(this->lines[i], max_width, FS_MONO);
+	if (!IsWidgetLowered(WID_TF_WRAPTEXT)) {
+		for (auto &line : this->lines) {
+			line.top = height;
+			height++;
+			line.bottom = height;
+		}
+	} else {
+		int max_width = this->GetWidget<NWidgetCore>(WID_TF_BACKGROUND)->current_x - WD_FRAMETEXT_LEFT - WD_FRAMERECT_RIGHT;
+		for (auto &line : this->lines) {
+			line.top = height;
+			height += GetStringHeight(line.text, max_width, FS_MONO) / FONT_HEIGHT_MONO;
+			line.bottom = height;
+		}
 	}
 
 	return height;
 }
 
+uint TextfileWindow::GetContentHeight()
+{
+	if (this->lines.size() == 0) return 0;
+	return this->lines.back().bottom;
+}
+
 /* virtual */ void TextfileWindow::UpdateWidgetSize(int widget, Dimension *size, const Dimension &padding, Dimension *fill, Dimension *resize)
 {
 	switch (widget) {
 		case WID_TF_BACKGROUND:
-			resize->height = 1;
+			resize->height = FONT_HEIGHT_MONO;
 
 			size->height = 4 * resize->height + TOP_SPACING + BOTTOM_SPACING; // At least 4 lines are visible.
 			size->width = std::max(200u, size->width); // At least 200 pixels wide.
@@ -104,18 +118,17 @@ uint TextfileWindow::GetContentHeight()
 }
 
 /** Set scrollbars to the right lengths. */
-void TextfileWindow::SetupScrollbars()
+void TextfileWindow::SetupScrollbars(bool force_reflow)
 {
 	if (IsWidgetLowered(WID_TF_WRAPTEXT)) {
-		this->vscroll->SetCount(this->GetContentHeight());
+		/* Reflow is mandatory if text wrapping is on */
+		uint height = this->ReflowContent();
+		this->vscroll->SetCount(std::min<uint>(UINT16_MAX, height));
 		this->hscroll->SetCount(0);
 	} else {
-		uint max_length = 0;
-		for (uint i = 0; i < this->lines.size(); i++) {
-			max_length = std::max(max_length, GetStringBoundingBox(this->lines[i], FS_MONO).width);
-		}
-		this->vscroll->SetCount((uint)this->lines.size() * FONT_HEIGHT_MONO);
-		this->hscroll->SetCount(max_length + WD_FRAMETEXT_LEFT + WD_FRAMETEXT_RIGHT);
+		uint height = force_reflow ? this->ReflowContent() : this->GetContentHeight();
+		this->vscroll->SetCount(std::min<uint>(UINT16_MAX, height));
+		this->hscroll->SetCount(this->max_length + WD_FRAMETEXT_LEFT + WD_FRAMETEXT_RIGHT);
 	}
 
 	this->SetWidgetDisabledState(WID_TF_HSCROLLBAR, IsWidgetLowered(WID_TF_WRAPTEXT));
@@ -126,7 +139,6 @@ void TextfileWindow::SetupScrollbars()
 	switch (widget) {
 		case WID_TF_WRAPTEXT:
 			this->ToggleWidgetLoweredState(WID_TF_WRAPTEXT);
-			this->SetupScrollbars();
 			this->InvalidateData();
 			break;
 	}
@@ -148,14 +160,18 @@ void TextfileWindow::SetupScrollbars()
 
 	/* Draw content (now coordinates given to DrawString* are local to the new clipping region). */
 	int line_height = FONT_HEIGHT_MONO;
-	int y_offset = -this->vscroll->GetPosition();
+	int pos = this->vscroll->GetPosition();
+	int cap = this->vscroll->GetCapacity();
+
+	for (auto &line : this->lines) {
+		if (line.bottom < pos) continue;
+		if (line.top > pos + cap) break;
 
-	for (uint i = 0; i < this->lines.size(); i++) {
+		int y_offset = (line.top - pos) * line_height;
 		if (IsWidgetLowered(WID_TF_WRAPTEXT)) {
-			y_offset = DrawStringMultiLine(0, right - x, y_offset, bottom - y, this->lines[i], TC_WHITE, SA_TOP | SA_LEFT, false, FS_MONO);
+			DrawStringMultiLine(0, right - x, y_offset, bottom - y, line.text, TC_WHITE, SA_TOP | SA_LEFT, false, FS_MONO);
 		} else {
-			DrawString(-this->hscroll->GetPosition(), right - x, y_offset, this->lines[i], TC_WHITE, SA_TOP | SA_LEFT, false, FS_MONO);
-			y_offset += line_height; // margin to previous element
+			DrawString(-this->hscroll->GetPosition(), right - x, y_offset, line.text, TC_WHITE, SA_TOP | SA_LEFT, false, FS_MONO);
 		}
 	}
 
@@ -167,7 +183,14 @@ void TextfileWindow::SetupScrollbars()
 	this->vscroll->SetCapacityFromWidget(this, WID_TF_BACKGROUND, TOP_SPACING + BOTTOM_SPACING);
 	this->hscroll->SetCapacityFromWidget(this, WID_TF_BACKGROUND);
 
-	this->SetupScrollbars();
+	this->SetupScrollbars(false);
+}
+
+/* virtual */ void TextfileWindow::OnInvalidateData(int data, bool gui_scope)
+{
+	if (!gui_scope) return;
+
+	this->SetupScrollbars(true);
 }
 
 /* virtual */ void TextfileWindow::Reset()
@@ -184,7 +207,7 @@ void TextfileWindow::SetupScrollbars()
 {
 	if (this->search_iterator >= this->lines.size()) return nullptr;
 
-	return this->lines[this->search_iterator++];
+	return this->lines[this->search_iterator++].text;
 }
 
 /* virtual */ bool TextfileWindow::Monospace()
@@ -364,14 +387,22 @@ static void Xunzip(byte **bufp, size_t *sizep)
 	str_validate(p, this->text + filesize, SVS_REPLACE_WITH_QUESTION_MARK | SVS_ALLOW_NEWLINE);
 
 	/* Split the string on newlines. */
-	this->lines.push_back(p);
+	int row = 0;
+	this->lines.emplace_back(row, p);
 	for (; *p != '\0'; p++) {
 		if (*p == '\n') {
 			*p = '\0';
-			this->lines.push_back(p + 1);
+			this->lines.emplace_back(++row, p + 1);
 		}
 	}
 
+	/* Calculate maximum text line length. */
+	uint max_length = 0;
+	for (auto &line : this->lines) {
+		max_length = std::max(max_length, GetStringBoundingBox(line.text, FS_MONO).width);
+	}
+	this->max_length = max_length;
+
 	CheckForMissingGlyphs(true, this);
 }