configurable display speedup by skipping refresh intervals (#835)

docs/mintty.1

@@ -1852,6 +1852,13 @@ The following settings appear neither in the options dialog nor as command line
 options, which means they can only be set in config files or using the
 \fB--option\fP or \fB-o\fP command line option.
 
+.TP
+\fBAccelerate display speed\fP (DisplaySpeedup=6)
+If mintty processes high volume of output, it will skip up to the given 
+number of refresh intervals (of 16ms each) in order to save output that 
+is visually scrolled off right away, so effectively increasing output speed.
+The maximum value is 9.
+
 .TP
 \fBClear selection highlighting on input\fP (ClearSelectionOnInput=true)
 When this is disabled, keyboard input or pasting does not clear selection highlighting.

src/config.c

@@ -153,6 +153,7 @@ const config default_cfg = {
   // "Hidden"
   .bidi = 2,
   .disable_alternate_screen = false,
+  .display_speedup = 6,
   .suppress_sgr = "",
   .suppress_dec = "",
   .suppress_win = "",
@@ -389,6 +390,7 @@ options[] = {
   // "Hidden"
   {"Bidi", OPT_INT, offcfg(bidi)},
   {"NoAltScreen", OPT_BOOL, offcfg(disable_alternate_screen)},
+  {"DisplaySpeedup", OPT_INT, offcfg(display_speedup)},
   {"SuppressSGR", OPT_STRING, offcfg(suppress_sgr)},
   {"SuppressDEC", OPT_STRING, offcfg(suppress_dec)},
   {"SuppressWIN", OPT_STRING, offcfg(suppress_win)},

src/config.h

@@ -152,6 +152,7 @@ typedef struct {
   // "Hidden"
   int bidi;
   bool disable_alternate_screen;
+  int display_speedup;
   string suppress_sgr;
   string suppress_dec;
   string suppress_win;

src/term.c

@@ -1059,6 +1059,8 @@ term_do_scroll(int topline, int botline, int lines, bool sb)
   bool down = lines < 0; // Scrolling downwards?
   lines = abs(lines);    // Number of lines to scroll by
 
+  lines_scrolled += lines;
+
   botline++; // One below the scroll region: easier to calculate with
 
   // Don't try to scroll more than the number of lines in the scroll region.

src/termout.c

@@ -2487,6 +2487,12 @@ term_do_write(const char *buf, uint len)
         if (hwc) // Previous high surrogate not followed by low one
           write_error();
 
+        // ASCII shortcut for some speedup (~5%), earliest applied here
+        if (wc >= ' ' && wc <= 0x7E && cset == CSET_ASCII) {
+          write_char(wc, 1);
+          continue;
+        }
+
         if (is_high_surrogate(wc)) {
           term.high_surrogate = wc;
           continue;

src/winpriv.h

@@ -45,6 +45,7 @@ extern int per_monitor_dpi_aware;
 
 extern bool click_focus_token;
 extern pos last_pos;
+extern int lines_scrolled;
 
 extern void win_flush_background(bool clearbg);
 extern void win_paint(void);

src/wintext.c

@@ -881,6 +881,9 @@ static HDC dc;
 static enum { UPDATE_IDLE, UPDATE_BLOCKED, UPDATE_PENDING } update_state;
 static bool ime_open;
 
+static int update_skipped = 0;
+int lines_scrolled = 0;
+
 #define dont_debug_cursor 1
 
 static struct charnameentry {
@@ -1105,14 +1108,27 @@ do_update(void)
 #if defined(debug_cursor) && debug_cursor > 1
   printf("do_update cursor_on %d @%d,%d\n", term.cursor_on, term.curs.y, term.curs.x);
 #endif
-  show_curchar_info('u');
   if (update_state == UPDATE_BLOCKED) {
     update_state = UPDATE_IDLE;
     return;
   }
 
+  update_skipped++;
+  int output_speed = lines_scrolled / term.rows;
+  lines_scrolled = 0;
+  if (update_skipped < cfg.display_speedup && cfg.display_speedup < 10
+      && output_speed > update_skipped
+     )
+  {
+    win_set_timer(do_update, 16);
+    return;
+  }
+  update_skipped = 0;
+
   update_state = UPDATE_BLOCKED;
 
+  show_curchar_info('u');
+
   dc = GetDC(wnd);
 
   win_paint_exclude_search(dc);