A PDF page taller than one raster is drawn at the size it is shown, in bands around the screen, not squeezed into 4096 rows and blown up

The render geometry (pdf_bridge.c) clamped a page's LONGEST side to
max_dimension: 4096 in the SDL policy, 1200 in Kitty's. A page 841.89 x
4818.9 pt, fit to a 1570 px wide pane, came out 715 x 4096 and was drawn
2.2 times too large at 1x, 4.4 times at 2x; Kitty's raster was 210 px
wide. Every request now caps a raster ROW at max_dimension, and caps
the height too only while that keeps the page at least as large as it
is shown (RenderRequest.display_width/height, set from the fit). A
tall page fit to width is rasterized at the pane's own width, in SDL,
and at Kitty's 96 dpi as any other page is there.

Such a page is not one buffer. A raster over 8192 rows (a texture's and
Kitty's 10000-pixel limit) or over 64 MiB is banded (pdf_view.zig): its
slot holds the rows on screen and a screenful either side, in 512-row
chunks, and is rendered again, keeping the chunks it has, once the
screen comes within half a screenful of its edge. Each chunk comes from
its own render reaching 32 rows past both edges, so a row of the page
is the same however it was scrolled to, and no chunk shows its clip
edge: band rows are within 3 levels of a whole-page render (108 pixels
of 45 million on the reported PDF). Highlights paint over the band's
clean rows as over a page's, and the band uses the page's CTM, so
search, selection and pointer geometry are unchanged.

A display-list render now culls to the band (a scissor in page space).
No pixel changes, but a frame that renders new rows of the reported
page at 2x fell from 33-43 ms to 13-16 ms (pdf-scroll-bench, --cell
16x32). design.pdf's scroll-bench pixels are identical.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 17 insertions, 2 deletions

diff --git a/src/pdf_bridge.h b/src/pdf_bridge.h
index e3514437..29d87bff 100644
--- a/src/pdf_bridge.h
+++ b/src/pdf_bridge.h
@@ -183,7 +183,10 @@ enum {
     PARDES_PDF_MAX_OUTLINE_ITEMS = 8192,
     PARDES_PDF_MAX_OUTLINE_DEPTH = 64,
     PARDES_PDF_MAX_OUTLINE_BYTES = 4 * 1024 * 1024,
-    PARDES_PDF_MAX_LINK_URI_BYTES = 64 * 1024
+    PARDES_PDF_MAX_LINK_URI_BYTES = 64 * 1024,
+
+    /* No page raster is taller than this, whatever it is shown at. */
+    PARDES_PDF_MAX_RASTER_ROWS = 1 << 20
 };
 
 pardes_pdf_document *pardes_pdf_open(const char *path, int *page_count);
@@ -199,7 +202,11 @@ int pardes_pdf_get_page_size(
 
 /*
  * Start at dpi, raise the uniform scale until both optional minimum pixel
- * dimensions are met, then clamp the page's longest side to max_dimension.
+ * dimensions are met, then clamp the page's width to max_dimension, and its
+ * height too unless that would draw it smaller than the optional display size
+ * (the width or height it is shown at): a tall page is then as tall as it is
+ * shown, and its owner renders it in bands. Every function below takes the
+ * same request and computes the same raster from it.
  * Measuring may populate the page cache, but never advances adaptive display
  * list promotion; only a successful render_into does that.
  */
@@ -210,6 +217,8 @@ int pardes_pdf_measure_render(
     int minimum_width,
     int minimum_height,
     int max_dimension,
+    int display_width,
+    int display_height,
     pardes_pdf_raster_layout *out
 );
 
@@ -233,6 +242,8 @@ int pardes_pdf_render_into(
     int minimum_width,
     int minimum_height,
     int max_dimension,
+    int display_width,
+    int display_height,
     const pardes_pdf_highlight *highlights,
     size_t highlight_count,
     unsigned char *samples,
@@ -257,6 +268,8 @@ int pardes_pdf_paint_highlights(
     int minimum_width,
     int minimum_height,
     int max_dimension,
+    int display_width,
+    int display_height,
     const pardes_pdf_highlight *highlights,
     size_t highlight_count,
     unsigned char *samples,
@@ -280,6 +293,8 @@ int pardes_pdf_highlight_rows(
     int minimum_width,
     int minimum_height,
     int max_dimension,
+    int display_width,
+    int display_height,
     const pardes_pdf_highlight *highlights,
     size_t highlight_count,
     int *row_start,