From 0872c9149b93e0c905dfcf689e97bb1aa78638a4 Mon Sep 17 00:00:00 2001
From: "Pedro M. de Echanove Pasquin" <pedro.echanove@tuxum.com>
Date: Wed, 4 Nov 2026 13:13:47 +0100
Subject: [PATCH] =?UTF-8?q?Make=20a=20scroll=20or=20fading=20viewport's=20?=
 =?UTF-8?q?sub-canvas=20again=20when=20the=20window's=20scale=20changes,?=
 =?UTF-8?q?=20rather=20than=20keep=20drawing=20it=20at=20the=20old=20one?=
 =?UTF-8?q?=20Prompt=20opened=20on=20a=20screen=20at=20scale=202=20and=20m?=
 =?UTF-8?q?oved=20to=20one=20at=20scale=201=20drew=20its=20tab=20labels=20?=
 =?UTF-8?q?and=20icons=20at=20twice=20their=20size,=20with=20only=20a=20co?=
 =?UTF-8?q?uple=20of=20letters=20fitting=20in=20each=20tab,=20while=20the?=
 =?UTF-8?q?=20star,=20the=20+=20and=20the=20menu=20beside=20the=20tab=20ro?=
 =?UTF-8?q?w,=20and=20the=20terminal,=20came=20out=20right.=20Opened=20on?=
 =?UTF-8?q?=20the=20second=20screen=20directly,=20the=20tabs=20were=20righ?=
 =?UTF-8?q?t=20too.=20The=20sub-canvases=20of=20scrolls=20(`scroll=5Fcanva?=
 =?UTF-8?q?ses`)=20and=20fading=20viewports=20(`viewport=5Fcanvases`)=20ar?=
 =?UTF-8?q?e=20kept=20from=20one=20frame=20to=20the=20next=20and=20reused,?=
 =?UTF-8?q?=20resized=20when=20their=20size=20changes.=20`Canvas::sub=5Fca?=
 =?UTF-8?q?nvas`=20copies=20the=20parent's=20`dpi=5Fscale`=20and=20layout?=
 =?UTF-8?q?=20viewport=20when=20it=20makes=20one,=20so=20a=20sub-canvas=20?=
 =?UTF-8?q?kept=20across=20a=20change=20of=20scale=20went=20on=20with=20th?=
 =?UTF-8?q?e=20old=20one.=20Prompt's=20tab=20strip=20sits=20in=20a=20horiz?=
 =?UTF-8?q?ontal=20scroll,=20so=20its=20labels=20were=20drawn=20at=20px(13?=
 =?UTF-8?q?)=20=C3=97=202=20on=20a=20canvas=20at=20scale=201.=20`reuse=5Fs?=
 =?UTF-8?q?ub=5Fcanvas`=20takes=20the=20pooled=20sub-canvas=20only=20while?=
 =?UTF-8?q?=20its=20`dpi=5Fscale`=20matches=20the=20parent's,=20resizing?=
 =?UTF-8?q?=20it=20as=20before,=20and=20otherwise=20makes=20a=20new=20one?=
 =?UTF-8?q?=20with=20`parent.sub=5Fcanvas`,=20which=20takes=20the=20curren?=
 =?UTF-8?q?t=20scale,=20density=20and=20viewport.=20Both=20the=20scroll=20?=
 =?UTF-8?q?and=20the=20fading=20viewport=20go=20through=20it.=20Not=20veri?=
 =?UTF-8?q?fied:=20the=20move=20of=20Prompt=20between=20the=20two=20screen?=
 =?UTF-8?q?s=20has=20not=20been=20tried=20with=20this=20change.?=
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---
 src/draw/layout.rs | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/src/draw/layout.rs b/src/draw/layout.rs
index ac21777..61ea6a1 100644
--- a/src/draw/layout.rs
+++ b/src/draw/layout.rs
@@ -17,6 +17,20 @@ use crate::widget::{ Element, LaidOutWidget, WidgetHandlers };
 use super::DrawCtx;
 use super::damage::clamp_rect_to;
 
+/// The previous frame's sub-canvas resized to `width × height`, or a new one when the parent's scale changed since, as a sub-canvas keeps the scale it was made with.
+fn reuse_sub_canvas( pooled: Option<Canvas>, parent: &Canvas, width: u32, height: u32 ) -> Canvas
+{
+	match pooled
+	{
+		Some( mut sub ) if sub.dpi_scale() == parent.dpi_scale() =>
+		{
+			if sub.size() != ( width, height ) { sub.resize( width, height ); }
+			sub
+		}
+		_ => parent.sub_canvas( width, height ),
+	}
+}
+
 pub( crate ) fn layout_and_draw<Msg: Clone>(
 	element:  &Element<Msg>,
 	canvas:   &mut Canvas,
@@ -295,13 +309,9 @@ pub( crate ) fn layout_and_draw<Msg: Clone>(
 				ctx.scroll_offsets.insert( my_idx, ( ox, oy ) );
 			}
 
-			// Reuse the sub-canvas from the previous frame if its size matches;
-			// only reallocate when the viewport is resized.
 			let sw  = (rect.width.ceil()  as u32).max( 1 );
 			let sh  = (rect.height.ceil() as u32).max( 1 );
-			let mut sub = ctx.scroll_canvases.remove( &my_idx )
-				.map( |mut c| { if c.size() != ( sw, sh ) { c.resize( sw, sh ); } c } )
-				.unwrap_or_else( || canvas.sub_canvas( sw, sh ) );
+			let mut sub = reuse_sub_canvas( ctx.scroll_canvases.remove( &my_idx ), canvas, sw, sh );
 			sub.clear();
 
 			// Child content fills at least the full viewport on each
@@ -462,9 +472,7 @@ pub( crate ) fn layout_and_draw<Msg: Clone>(
 			let vh = ( rect.height.ceil() as u32 ).max( 1 );
 			let seq = ctx.viewport_seq;
 			ctx.viewport_seq += 1;
-			let mut sub = ctx.viewport_canvases.remove( &seq )
-				.map( |mut c| { if c.size() != ( vw, vh ) { c.resize( vw, vh ); } c } )
-				.unwrap_or_else( || canvas.sub_canvas( vw, vh ) );
+			let mut sub = reuse_sub_canvas( ctx.viewport_canvases.remove( &seq ), canvas, vw, vh );
 			if v.local_viewport { sub.set_local_layout_viewport(); }
 			sub.clear();