Add lyxrc option to force the use of backing store

LyX relies on a a backing store to draw when running under macOS or
Wayland, because Qt arbitrarily overwrites parts of the workarea
before we paint (and we paint only the parts that need to be painted).
However it seems that this is also necessary on X11 when the WM theme
is translucid. Since there is no way that I know of to detect this
situation, this patch adds a LyXRC setting to manually select this
drawing strategy.

Note that using a backing store is not always a good solution, since
this disables subpixel aliasing.

At this point there is no UI for the variable.

Fixes bug #12119

src/LyXRC.cpp

@@ -107,6 +107,7 @@ LexerKeyword lyxrcTags[] = {
 	{ "\\dialogs_iconify_with_main", LyXRC::RC_DIALOGS_ICONIFY_WITH_MAIN },
 	{ "\\display_graphics", LyXRC::RC_DISPLAY_GRAPHICS },
 	{ "\\document_path", LyXRC::RC_DOCUMENTPATH },
+	{ "\\draw_strategy", LyXRC::RC_DRAW_STRATEGY },
 	{ "\\editor_alternatives", LyXRC::RC_EDITOR_ALTERNATIVES },
 	{ "\\escape_chars", LyXRC::RC_ESC_CHARS },
 	{ "\\example_path", LyXRC::RC_EXAMPLEPATH },
@@ -1134,6 +1135,19 @@ LyXRC::ReturnValues LyXRC::read(Lexer & lexrc, bool check_format)
 			}
 			break;
 
+		case RC_DRAW_STRATEGY:
+			if (lexrc.next()) {
+				string const tmp = lexrc.getString();
+				if (tmp == "partial")
+					draw_strategy = DS_PARTIAL;
+				else if (tmp == "backingstore")
+					draw_strategy = DS_BACKINGSTORE;
+				else {
+					draw_strategy = DS_PARTIAL;
+					LYXERR0("Unrecognized draw strategy " << tmp <<'"');
+				}
+			}
+			break;
 
 		case RC_LAST:
 			break; // this is just a dummy
@@ -2021,6 +2035,24 @@ void LyXRC::write(ostream & os, bool ignore_system_lyxrc, string const & name) c
 		}
 		if (tag != RC_LAST)
 			break;
+		// fall through
+	case RC_DRAW_STRATEGY:
+		if (ignore_system_lyxrc ||
+			draw_strategy != system_lyxrc.draw_strategy) {
+			string status;
+			switch (draw_strategy) {
+			case DS_PARTIAL:
+				status = "partial";
+				break;
+			case DS_BACKINGSTORE:
+				status = "backingstore";
+				break;
+			}
+			os << "\\draw_strategy " << status << '\n';
+		}
+		if (tag != RC_LAST)
+			break;
+		// fall through
 
 	os << "\n#\n"
 		<< "# COLOR SECTION ###################################\n"
@@ -2910,6 +2942,7 @@ void actOnUpdatedPrefs(LyXRC const & lyxrc_orig, LyXRC const & lyxrc_new)
 				package().document_dir() = FileName(lyxrc.document_path);
 		}
 		// fall through
+	case LyXRC::RC_DRAW_STRATEGY:
 	case LyXRC::RC_EDITOR_ALTERNATIVES:
 	case LyXRC::RC_ESC_CHARS:
 	case LyXRC::RC_EXAMPLEPATH:

src/LyXRC.h

@@ -189,6 +189,7 @@ public:
 		RC_VIEWER_ALTERNATIVES,
 		RC_VISUAL_CURSOR,
 		RC_CLOSE_BUFFER_WITH_LAST_VIEW,
+		RC_DRAW_STRATEGY,
 		RC_LAST
 	};
 
@@ -573,6 +574,17 @@ public:
 
 	///
 	BookmarksVisibility bookmarks_visibility = BMK_NONE;
+
+	enum DrawStrategy {
+		// draw all (not implemented yet)
+		// FS_FULL,
+		// draw only what has changed
+		DS_PARTIAL,
+		// draw in backing store (only what has changed)
+		DS_BACKINGSTORE
+	};
+	///
+	DrawStrategy draw_strategy = DS_PARTIAL;
 };
 
 

src/frontends/qt/GuiApplication.cpp

@@ -2710,6 +2710,18 @@ Menus & GuiApplication::menus()
 }
 
 
+bool GuiApplication::needsBackingStore() const
+{
+	/* Qt on macOS and Wayland does not respect the
+	 * Qt::WA_OpaquePaintEvent attribute and resets the widget backing
+	 * store at each update. Therefore, we use our own backing store
+	 * in these two cases. It is also possible to force the use of the
+	 * backing store for cases like x11 with transparent WM themes.
+	 */
+	return platformName() == "cocoa" || platformName().contains("wayland");
+}
+
+
 QList<int> GuiApplication::viewIds() const
 {
 	return d->views_.keys();

src/frontends/qt/GuiApplication.h

@@ -110,6 +110,9 @@ public:
 	///
 	Menus & menus();
 
+	/// \returns true if painting the workarea requires a backing store.
+	bool needsBackingStore() const;
+
 	/// \name Methods inherited from QApplication class
 	//@{
 	bool notify(QObject * receiver, QEvent * event) override;

src/frontends/qt/GuiWorkArea.cpp

@@ -132,13 +132,10 @@ SyntheticMouseEvent::SyntheticMouseEvent()
 GuiWorkArea::Private::Private(GuiWorkArea * parent)
 	: p(parent), completer_(new GuiCompleter(p, p))
 {
-	/* Qt on macOS and Wayland does not respect the
-	 * Qt::WA_OpaquePaintEvent attribute and resets the widget backing
-	 * store at each update. Therefore, we use our own backing store
-	 * in these two cases.
-	 */
-	use_backingstore_ = guiApp->platformName() == "cocoa"
-		|| guiApp->platformName().contains("wayland");
+	use_backingstore_ = lyxrc.draw_strategy == LyXRC::DS_BACKINGSTORE
+		|| guiApp->needsBackingStore();
+	LYXERR(Debug::WORKAREA, "Drawing strategy is: "
+	                        << (use_backingstore_ ? "backingstore" : "partial"));
 
 	int const time = QApplication::cursorFlashTime() / 2;
 	if (time > 0) {