fix(dirvish): stop webm preview thumbnails flashing blank

dirvish keys its media thumbnail cache on the exact preview-window pixel width (thumbnails/<width>/<md5>.jpg). Fast image thumbnails finish before the window settles, so their width is stable. Slow webm generation loses that race: the jpg is written at one width, then redisplay recomputes the width, and jitter across a floor boundary looks it up in a different bucket, so it misses, regenerates, and flashes blank.

I quantize the computed size to a 100px bucket (cj/dirvish-thumb-width-bucket) so jitter maps to one stable cache key. It's a filter-return advice on dirvish-media--img-size, added once dirvish-widgets loads. It's a local workaround for the upstream cache design. If the settle swing turns out larger than one bucket, widen the bucket or fall back to a width-independent hash.

The quantize math is unit-tested. The live preview render needs a manual check.

1 files changed, 60 insertions, 0 deletions

diff --git a/tests/test-dirvish-config--quantize-thumb-size.el b/tests/test-dirvish-config--quantize-thumb-size.el
new file mode 100644
index 00000000..a26ef090
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+++ b/tests/test-dirvish-config--quantize-thumb-size.el
@@ -0,0 +1,60 @@
+;;; test-dirvish-config--quantize-thumb-size.el --- thumbnail width-bucket tests -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; `cj/--dirvish-quantize-thumb-size' rounds dirvish's computed thumbnail size to
+;; a coarse pixel bucket so small preview-window jitter maps to one stable cache
+;; key instead of a fresh miss + regenerate (the webm thumbnail-flash bug).  Pure
+;; math; the :filter-return advice that wires it onto `dirvish-media--img-size'
+;; is verified live.
+
+;;; Code:
+
+(require 'ert)
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'dirvish-config)
+
+(declare-function cj/--dirvish-quantize-thumb-size "dirvish-config" (size bucket))
+
+;;; ------------------------------- rounding -----------------------------------
+
+(ert-deftest test-dirvish-quantize-rounds-down ()
+  "Normal: a size below the bucket midpoint rounds down to the bucket."
+  (should (= (cj/--dirvish-quantize-thumb-size 931 100) 900)))
+
+(ert-deftest test-dirvish-quantize-rounds-up ()
+  "Normal: a size above the bucket midpoint rounds up to the next bucket."
+  (should (= (cj/--dirvish-quantize-thumb-size 552 100) 600)))
+
+(ert-deftest test-dirvish-quantize-on-bucket-unchanged ()
+  "Normal: a size already on a bucket boundary is returned unchanged."
+  (should (= (cj/--dirvish-quantize-thumb-size 600 100) 600)))
+
+(ert-deftest test-dirvish-quantize-jitter-maps-to-one-key ()
+  "Boundary: nearby sizes within a bucket collapse to the same cache key."
+  (should (= (cj/--dirvish-quantize-thumb-size 931 100)
+             (cj/--dirvish-quantize-thumb-size 949 100))))
+
+(ert-deftest test-dirvish-quantize-float-input ()
+  "Boundary: a float size (pre-floor) still yields an integer bucket."
+  (let ((r (cj/--dirvish-quantize-thumb-size 931.4 100)))
+    (should (integerp r))
+    (should (= r 900))))
+
+;;; ------------------------------- clamping -----------------------------------
+
+(ert-deftest test-dirvish-quantize-tiny-clamps-to-bucket ()
+  "Boundary: a size that would round to 0 clamps up to one bucket, never 0."
+  (should (= (cj/--dirvish-quantize-thumb-size 1 100) 100)))
+
+;;; ------------------------------ disabled path -------------------------------
+
+(ert-deftest test-dirvish-quantize-zero-bucket-passthrough ()
+  "Error/disabled: a zero bucket returns the size unchanged."
+  (should (= (cj/--dirvish-quantize-thumb-size 931 0) 931)))
+
+(ert-deftest test-dirvish-quantize-nil-bucket-passthrough ()
+  "Error/disabled: a nil bucket returns the size unchanged."
+  (should (= (cj/--dirvish-quantize-thumb-size 931 nil) 931)))
+
+(provide 'test-dirvish-config--quantize-thumb-size)
+;;; test-dirvish-config--quantize-thumb-size.el ends here