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Metal: defer presenting clear-only update cycles (first tab-switch flash)

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Switching to a tab whose buffer faces are not realized yet garbages the
frame: redisplay first runs an update cycle that only calls clear_frame,
and Metal presented that blank frame, flashing white for the ~30 ms the
follow-up cycle needs to realize faces and repaint.  NS does not flash
because AppKit coalesces backing-store flushes within the event cycle.

Track per-cycle whether clear_frame ran and whether any glyph string was
drawn; a clear-only cycle commits to the static texture but defers the
present (needsPresent), which the follow-up content cycle or
flush_display then picks up.  Also adds MTL_LOG_SEQ=1 tracing of the
begin/clear/draw/end/present sequence, which is what exposed the blank
present.
This commit is contained in:
Andros Fenollosa 2026-06-03 22:30:04 +02:00
parent 748a2cd107
commit 93aa0e5d32
2 changed files with 41 additions and 1 deletions
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8
src/mtlterm.h
View file

@ -179,6 +179,14 @@ typedef struct mtl_spring {
drawRect: expose path for the uncovered pixels; Metal has none). */
@property (nonatomic, assign) int lastMiniHeight;
/* Within the current update cycle: clear_frame ran / real content was drawn.
A cycle that only cleared (a garbaged frame, e.g. the first switch to a tab
whose faces are not realized yet) must NOT present, or the user sees a blank
flash before the follow-up cycle paints the actual content. NS does not
flash there because AppKit coalesces the backing-store flushes. */
@property (nonatomic, assign) BOOL cycleSawClear;
@property (nonatomic, assign) BOOL cycleSawDraw;
/* Main Emacs render cycle (renders to staticTexture) */
- (void)beginFrame;
- (void)endFrame;

34
src/mtlterm.m
View file

@ -1400,6 +1400,20 @@ easing_apply (MtlScrollEasing mode, float t)
self.needsPresent = YES;
}
/* Sequence tracing for present-flow debugging (MTL_LOG_SEQ=1). */
static BOOL
mtl_log_seq_p (void)
{
static int on = -1;
if (on < 0) on = getenv ("MTL_LOG_SEQ") != NULL;
return on > 0;
}
#define MTL_SEQ(fmt, ...) \
do { if (mtl_log_seq_p ()) \
fprintf (stderr, "[mtlseq %.3f] " fmt "\n", \
CACurrentMediaTime (), ##__VA_ARGS__); } while (0)
- (void)compositeToScreen
{
if (!self.staticTexture || !g_blit_pipeline) return;
@ -1407,6 +1421,8 @@ easing_apply (MtlScrollEasing mode, float t)
id<CAMetalDrawable> drawable = [self.metalLayer nextDrawable];
if (!drawable) return;
MTL_SEQ ("PRESENT");
self.needsPresent = NO; /* about to present whatever is in the static texture */
NSSize sz = self.metalLayer.frame.size;
@ -2709,12 +2725,14 @@ mtl_draw_glyph_string (struct glyph_string *s)
if (fd.encoder)
{
fd.cycleSawDraw = YES;
[fd applyClipRect:clip];
mtl_draw_glyph_string_impl (s);
[fd clearClipRect];
}
else
{
MTL_SEQ ("immediate glyph draw x=%d y=%d w=%d", s->x, s->y, s->width);
[fd beginFrame];
[fd applyClipRect:clip];
mtl_draw_glyph_string_impl (s);
@ -2737,6 +2755,8 @@ mtl_clear_frame (struct frame *f)
committed immediate frame with a deferred present, like
mtl_draw_glyph_string. */
BOOL immediate = (fd.encoder == nil);
MTL_SEQ ("clear_frame (immediate=%d)", immediate);
fd.cycleSawClear = YES;
if (immediate) [fd beginFrame];
NSRect bounds = fd.metalLayer ? CGRectMake (0, 0,
@ -2828,6 +2848,7 @@ mtl_flush_display (struct frame *f)
encoder that never got endEncoding, causing an assertion failure. */
MtlFrameData *fd = mtl_get_frame_data (f);
if (!fd) return;
MTL_SEQ ("flush_display (encoder=%d needsPresent=%d)", fd.encoder != nil, (int) fd.needsPresent);
if (fd.encoder)
[fd endFrame];
else if (fd.needsPresent)
@ -2844,6 +2865,9 @@ mtl_update_begin (struct frame *f)
/* Guard: if encoder already active (e.g. from a re-entrant redisplay),
end it cleanly before starting a new frame. */
if (fd.encoder) [fd endFrame];
MTL_SEQ ("update_begin");
fd.cycleSawClear = NO;
fd.cycleSawDraw = NO;
[fd beginFrame];
/* Expose substitute: when the minibuffer (echo area) changes height, the
@ -2874,7 +2898,15 @@ static void
mtl_update_end (struct frame *f)
{
MtlFrameData *fd = mtl_get_frame_data (f);
if (fd) [fd endFrame];
if (!fd) return;
/* A cycle that only cleared the garbaged frame (no content drawn) commits
to the static texture but defers the present: presenting it would flash
a blank frame for the tens of ms the follow-up cycle needs to realize
faces/fonts and repaint (seen on the first switch to a new tab). The
deferred present is picked up by the next cycle or by flush_display. */
BOOL clearOnly = fd.cycleSawClear && !fd.cycleSawDraw;
MTL_SEQ ("update_end%s", clearOnly ? " (clear-only, present deferred)" : "");
[fd endFramePresent:!clearOnly];
}
static void