rustdoc: add a handle that makes sidebar resizing more obvious

This change is based on some discussion on [lolbinarycat's idea],
but with a more "traditional" design. Specifically, this is the
closest thing I could find to a consensus across many systems I
looked at for inspiration:

- In Jira, resizable sidebars have a stack of four dots.
- In The GIMP, resizable sidebars have a stack of three dots.
- In [old Windows], "panes" are defined to have the same border
  style as a window, which has a raised appearance.
- In [NeXT], a drag point usually had an innie, whether the line in a
  slider or the circle in a scroller; I can also hide and show the
  favorites bar in Workspace by dragging on a circular "grip spot"
- In [old Mac], drag handles for things usually had a "grip track"
  of parallel lines.
- [OSX] kept that, but the "Source List" part of the Finder still had
  the circle grip for a time the same way Workspace did

[lolbinarycat's idea]: rust-lang#139420
[old Windows]: https://archive.org/details/windowsinterface00micr/page/n9/mode/2up
[old Mac]: https://archive.org/details/apple-hig/1996_Human_Interface_Guidelines_for_Mac_OS_8_%28WWDC_Release%29/page/16/mode/2up
[NeXT]: https://archive.org/details/apple-hig/1993%20NeXTSTEP%20User%20Interface%20Guidelines%20-%20Release%203/page/145/mode/2up
[OSX]: https://dn721903.ca.archive.org/0/items/apple-hig/MacOSX_HIG_2005_09_08.pdf#page=267

Author: notriddle

src/librustdoc/html/static/css/noscript.css

@@ -43,6 +43,7 @@ nav.sub {
 	--settings-button-border-focus: #717171;
 	--sidebar-background-color: #f5f5f5;
 	--sidebar-background-color-hover: #e0e0e0;
+	--sidebar-border-color: #ddd;
 	--code-block-background-color: #f5f5f5;
 	--scrollbar-track-background-color: #dcdcdc;
 	--scrollbar-thumb-background-color: rgba(36, 37, 39, 0.6);
@@ -135,6 +136,8 @@ nav.sub {
 	--scrape-example-code-wrapper-background-end: rgba(255, 255, 255, 0);
 	--sidebar-resizer-hover: hsl(207, 90%, 66%);
 	--sidebar-resizer-active: hsl(207, 90%, 54%);
+	--sidebar-resizer-img-filter: opacity(66%);
+	--sidebar-resizer-img-hover-filter: none;
 }
 /* End theme: light */
 
@@ -149,6 +152,7 @@ nav.sub {
 		--settings-button-border-focus: #ffb900;
 		--sidebar-background-color: #505050;
 		--sidebar-background-color-hover: #676767;
+		--sidebar-border-color: #2A2A2A;
 		--code-block-background-color: #2A2A2A;
 		--scrollbar-track-background-color: #717171;
 		--scrollbar-thumb-background-color: rgba(32, 34, 37, .6);
@@ -244,6 +248,8 @@ nav.sub {
 		--scrape-example-code-wrapper-background-end: rgba(53, 53, 53, 0);
 		--sidebar-resizer-hover: hsl(207, 30%, 54%);
 		--sidebar-resizer-active: hsl(207, 90%, 54%);
+		--sidebar-resizer-img-filter: opacity(66%);
+		--sidebar-resizer-img-hover-filter: none;
 	}
 /* End theme: dark */
 }

src/librustdoc/html/static/css/rustdoc.css

@@ -1,4 +1,6 @@
-/* When static files are updated, their suffixes need to be updated.
+/*
+// ignore-tidy-filelength
+	When static files are updated, their suffixes need to be updated.
 	1. In the top directory run:
 		./x.py doc --stage 1 library/core
 	2. Find the directory containing files named with updated suffixes:
@@ -493,12 +495,13 @@ img {
 	top: 0;
 	left: 0;
 	z-index: var(--desktop-sidebar-z-index);
+	/* resize indicator: hide this when on touch or mobile */
+	border-right: solid 1px var(--sidebar-border-color);
 }
 
 .rustdoc.src .sidebar {
 	flex-basis: 50px;
 	width: 50px;
-	border-right: 1px solid;
 	overflow-x: hidden;
 	/* The sidebar is by default hidden  */
 	overflow-y: hidden;
@@ -516,8 +519,28 @@ img {
 	z-index: calc(var(--desktop-sidebar-z-index) + 1);
 	position: fixed;
 	height: 100%;
-	/* make sure there's a 1px gap between the scrollbar and resize handle */
-	left: calc(var(--desktop-sidebar-width) + 1px);
+	left: var(--desktop-sidebar-width);
+	display: flex;
+	align-items: center;
+	justify-content: center;
+}
+
+.sidebar-resizer::after {
+	content: url('data:image/svg+xml,\
+		<svg xmlns="http://www.w3.org/2000/svg" width="8" height="24" viewBox="0 0 8 24"> \
+		<linearGradient id="x" x1="0" x2="0" y1="0" y2="1"> \
+		<stop offset="0.5" stop-color="%23666"/><stop offset="0.5" stop-color="%23ccc"/> \
+		</linearGradient> \
+		<circle r="2" fill="none" stroke-width="2" stroke="url(%23x)" cy="21" cx="3"/> \
+		<circle r="2" fill="none" stroke-width="2" stroke="url(%23x)" cy="15" cx="3"/> \
+		<circle r="2" fill="none" stroke-width="2" stroke="url(%23x)" cy="9" cx="3"/> \
+		<circle r="2" fill="none" stroke-width="2" stroke="url(%23x)" cy="3" cx="3"/></svg>');
+	width: 8px;
+	height: 24px;
+	filter: var(--sidebar-resizer-img-filter);
+}
+.sidebar-resizer:hover::after {
+	filter: var(--sidebar-resizer-img-hover-filter);
 }
 
 .rustdoc.src .sidebar-resizer {
@@ -540,7 +563,7 @@ img {
 }
 
 .sidebar-resizing * {
-	cursor: col-resize !important;
+	cursor: ew-resize !important;
 }
 
 .sidebar-resizing .sidebar {
@@ -558,7 +581,7 @@ img {
 	margin: 0;
 	/* when active or hovered, place resizer glow on top of the sidebar (right next to, or even
 	   on top of, the scrollbar) */
-	left: var(--desktop-sidebar-width);
+	left: calc(var(--desktop-sidebar-width) - 1px);
 	border-left: solid 1px var(--sidebar-resizer-hover);
 }
 
@@ -575,6 +598,10 @@ img {
 		/* too easy to hit the resizer while trying to hit the [-] toggle */
 		display: none !important;
 	}
+	.sidebar {
+		/* resize indicator: hide this when on touch or mobile */
+		border-right: none;
+	}
 }
 
 .sidebar-resizer.active {
@@ -585,8 +612,10 @@ img {
 	margin-left: -140px;
 	border-left: none;
 }
-.sidebar-resizer.active:before {
+.sidebar-resizer.active::before {
 	border-left: solid 2px var(--sidebar-resizer-active);
+	margin-left: 8px;
+	padding-left: 1px;
 	display: block;
 	height: 100%;
 	content: "";
@@ -2510,6 +2539,8 @@ in src-script.js and main.js
 		/* Reduce height slightly to account for mobile topbar. */
 		height: calc(100vh - 45px);
 		width: 200px;
+		/* resize indicator: hide this when on touch or mobile */
+		border-right: none;
 	}
 
 	/* The source view uses a different design for the sidebar toggle, and doesn't have a topbar,
@@ -2893,6 +2924,7 @@ by default.
 	--settings-button-border-focus: #717171;
 	--sidebar-background-color: #f5f5f5;
 	--sidebar-background-color-hover: #e0e0e0;
+	--sidebar-border-color: #ddd;
 	--code-block-background-color: #f5f5f5;
 	--scrollbar-track-background-color: #dcdcdc;
 	--scrollbar-thumb-background-color: rgba(36, 37, 39, 0.6);
@@ -2985,6 +3017,8 @@ by default.
 	--scrape-example-code-wrapper-background-end: rgba(255, 255, 255, 0);
 	--sidebar-resizer-hover: hsl(207, 90%, 66%);
 	--sidebar-resizer-active: hsl(207, 90%, 54%);
+	--sidebar-resizer-img-filter: opacity(66%);
+	--sidebar-resizer-img-hover-filter: none;
 }
 /* End theme: light */
 
@@ -2998,6 +3032,7 @@ by default.
 	--settings-button-border-focus: #ffb900;
 	--sidebar-background-color: #505050;
 	--sidebar-background-color-hover: #676767;
+	--sidebar-border-color: #2A2A2A;
 	--code-block-background-color: #2A2A2A;
 	--scrollbar-track-background-color: #717171;
 	--scrollbar-thumb-background-color: rgba(32, 34, 37, .6);
@@ -3093,6 +3128,8 @@ by default.
 	--scrape-example-code-wrapper-background-end: rgba(53, 53, 53, 0);
 	--sidebar-resizer-hover: hsl(207, 30%, 54%);
 	--sidebar-resizer-active: hsl(207, 90%, 54%);
+	--sidebar-resizer-img-filter: opacity(66%);
+	--sidebar-resizer-img-hover-filter: none;
 }
 /* End theme: dark */
 
@@ -3110,6 +3147,7 @@ Original by Dempfi (https://github.com/dempfi/ayu)
 	--settings-button-border-focus: #e0e0e0;
 	--sidebar-background-color: #14191f;
 	--sidebar-background-color-hover: rgba(70, 70, 70, 0.33);
+	--sidebar-border-color: #000;
 	--code-block-background-color: #191f26;
 	--scrollbar-track-background-color: transparent;
 	--scrollbar-thumb-background-color: #5c6773;
@@ -3205,6 +3243,8 @@ Original by Dempfi (https://github.com/dempfi/ayu)
 	--scrape-example-code-wrapper-background-end: rgba(15, 20, 25, 0);
 	--sidebar-resizer-hover: hsl(34, 50%, 33%);
 	--sidebar-resizer-active: hsl(34, 100%, 66%);
+	--sidebar-resizer-img-filter: opacity(66%);
+	--sidebar-resizer-img-hover-filter: none;
 }
 
 :root[data-theme="ayu"] h1,