Prevent flickering in curses

Reading from curs_refresh(3X) from curses, calling doupdate() repeatedly
will cause 'several bursts of output to the screen'. wnoutrefresh() has
the smarts to only copy the changed lines to the copied virtual screen,
but doupdate() does not.

There have been several bug reports related to flickering but all seems
to be inconsistenly reproducible due to different terminal buffering
behavior. See #1032, #327

Unfortunately, when I am using a slow display, I still notice
flickering, so this commit changes the routines for opening new windows
and splitting windows to wait until the last change is finished before
calling doupdate().

ui-terminal-curses.c

@@ -230,7 +230,8 @@ static void ui_curses_blit(UiTerm *tui) {
 		}
 	}
 	wnoutrefresh(stdscr);
-	doupdate();
+	if (tui->doupdate)
+		doupdate();
 }
 
 static void ui_curses_clear(UiTerm *tui) {

ui-terminal.c

@@ -48,6 +48,7 @@ typedef struct {
 	CellStyle *styles;        /* each window has UI_STYLE_MAX different style definitions */
 	size_t cells_size;        /* #bytes allocated for 2D grid (grows only) */
 	Cell *cells;              /* 2D grid of cells, at least as large as current terminal size */
+	bool doupdate;            /* Whether to update the screen after refreshing contents */
 } UiTerm;
 
 struct UiTermWin {
@@ -347,6 +348,11 @@ static void ui_arrange(Ui *ui, enum UiLayout layout) {
 	}
 }
 
+static void ui_doupdates(Ui *ui, bool doupdate) {
+	UiTerm *tui = (UiTerm*)ui;
+	tui->doupdate = doupdate;
+}
+
 static void ui_draw(Ui *ui) {
 	debug("ui-draw\n");
 	UiTerm *tui = (UiTerm*)ui;
@@ -684,6 +690,7 @@ Ui *ui_term_new(void) {
 	}
 	tui->styles_size = styles_size;
 	tui->styles = styles;
+	tui->doupdate = true;
 	Ui *ui = (Ui*)tui;
 	*ui = (Ui) {
 		.init = ui_init,
@@ -699,6 +706,7 @@ Ui *ui_term_new(void) {
 		.draw = ui_draw,
 		.redraw = ui_redraw,
 		.arrange = ui_arrange,
+		.doupdates = ui_doupdates,
 		.die = ui_die,
 		.info = ui_info,
 		.info_hide = ui_info_hide,

ui.h

@@ -98,6 +98,7 @@ struct Ui {
 	void (*redraw)(Ui*);
 	void (*suspend)(Ui*);
 	void (*resume)(Ui*);
+	void (*doupdates)(Ui*, bool);
 	bool (*getkey)(Ui*, TermKeyKey*);
 	void (*terminal_save)(Ui*, bool fscr);
 	void (*terminal_restore)(Ui*);

vis.c

@@ -529,6 +529,7 @@ bool vis_window_reload(Win *win) {
 }
 
 bool vis_window_split(Win *original) {
+	vis_doupdates(original->vis, false);
 	Win *win = window_new_file(original->vis, original->file, UI_OPTION_STATUSBAR);
 	if (!win)
 		return false;
@@ -541,6 +542,7 @@ bool vis_window_split(Win *original) {
 	win->file = original->file;
 	view_options_set(win->view, view_options_get(original->view));
 	view_cursor_to(win->view, view_cursor_get(original->view));
+	vis_doupdates(win->vis, true);
 	return true;
 }
 
@@ -599,15 +601,21 @@ void vis_resume(Vis *vis) {
 	vis->ui->resume(vis->ui);
 }
 
+void vis_doupdates(Vis *vis, bool doupdate) {
+	vis->ui->doupdates(vis->ui, doupdate);
+}
+
 bool vis_window_new(Vis *vis, const char *filename) {
 	File *file = file_new(vis, filename);
 	if (!file)
 		return false;
+	vis_doupdates(vis, false);
 	Win *win = window_new_file(vis, file, UI_OPTION_STATUSBAR|UI_OPTION_SYMBOL_EOF);
 	if (!win) {
 		file_free(vis, file);
 		return false;
 	}
+	vis_doupdates(vis, true);
 
 	return true;
 }

vis.h

@@ -143,6 +143,13 @@ void vis_suspend(Vis*);
  * @endrst
  */
 void vis_resume(Vis*);
+/**
+ * Set doupdate flag.
+ * @rst
+ * .. note:: Prevent flickering in curses by delaying window updates.
+ * @endrst
+ */
+void vis_doupdates(Vis*, bool);
 /**
  * Inform the editor core that a signal occurred.
  * @return Whether the signal was handled.