fix(editor): keep selections whole over line backgrounds and indent guides (#1568)

* fix(editor): keep selections whole over line backgrounds and indent guides

Multi-line selections could show a 1px seam between their stacked
rectangles, indent guides showed as short dashes inside a selection,
and a breakpoint or execution line hid the selection past its text.

- Snap selection rectangle edges to device pixels so stacked pieces
  share one edge.
- Paint caret-row, breakpoint and execution line backgrounds on a
  pseudo-element below the selection layer.
- Draw indent guides on a pseudo-element above the text.

* test(e2e): check the visible selection layer in the theme flow

src/editor/core/cm/prefs.ts

@@ -35,6 +35,7 @@ import {
 } from "@codemirror/commands";
 import { markdownImages } from "./markdownImages";
 import { selectionMarks } from "./selectionMarks";
+import { seamlessSelection } from "./selectionLayer";
 import type { EditorExtras } from "@/platform/editorSettings";
 import { openideaHighlightStyle } from "./highlight";
 import {
@@ -506,6 +507,7 @@ export function caretExtension(prefs: EditorExtras): Extension {
     a.smoothCaretMovement ? caretGlide : [],
     drawSelection({ cursorBlinkRate: a.caretBlinking ? Math.max(100, Math.min(5000, a.caretBlinkPeriod)) * 2 : 0 }),
     selectionMarks,
+    seamlessSelection(),
     classes.length ? EditorView.editorAttributes.of({ class: classes.join(" ") }) : [],
   ];
 }

src/editor/core/cm/selectionLayer.test.ts

@@ -0,0 +1,21 @@
+import { describe, expect, it } from "vitest";
+import { RectangleMarker } from "@codemirror/view";
+import { snapMarkers } from "./selectionLayer";
+
+describe("snapMarkers", () => {
+  it("makes stacked selection rectangles share their edge exactly", () => {
+    // Middle piece's bottom (top + height) and the last line's top differ by float error around x.25.
+    const middle = new RectangleMarker("cm-selectionBackground", 6, 26.249999999, 500, 22.000000002);
+    const last = new RectangleMarker("cm-selectionBackground", 6, 48.25, 40, 22);
+    const [a, b] = snapMarkers([middle, last], "cm-selectionBackground", 2) as [RectangleMarker, RectangleMarker];
+    expect(a.top + a.height).toBe(b.top);
+  });
+
+  it("snaps edges to device pixels and keeps horizontal geometry", () => {
+    const m = snapMarkers([new RectangleMarker("x", 6.3, 10.3, 100, 21.9)], "x", 2)[0]!;
+    expect(m.top).toBe(10.5);
+    expect(m.top + m.height).toBe(32);
+    expect(m.left).toBe(6.3);
+    expect(m.width).toBe(100);
+  });
+});

src/editor/core/cm/selectionLayer.ts

@@ -0,0 +1,48 @@
+import { EditorView, RectangleMarker, layer } from "@codemirror/view";
+import type { Extension } from "@codemirror/state";
+
+/** Rounds a CSS length to the nearest device pixel. */
+export function snapToDevicePixel(v: number, dpr: number): number {
+  return Math.round(v * dpr) / dpr;
+}
+
+/**
+ * Re-emits selection rectangles with top and bottom edges snapped to device pixels. A multi-line
+ * selection is drawn as several stacked rectangles; their shared edge comes from separate float
+ * sums, and when it lands on a fractional pixel each side rounds differently and a 1px seam shows.
+ */
+export function snapMarkers(markers: readonly RectangleMarker[], className: string, dpr: number): RectangleMarker[] {
+  return markers.map((m) => {
+    const top = snapToDevicePixel(m.top, dpr);
+    const bottom = snapToDevicePixel(m.top + m.height, dpr);
+    return new RectangleMarker(className, m.left, top, m.width, bottom - top);
+  });
+}
+
+const CLASS = "cm-selectionBackground";
+
+const snappedSelectionLayer = layer({
+  above: false,
+  class: "cm-selectionLayer",
+  mount: (dom) => dom.classList.add("cm-openidea-selectionLayer"),
+  markers(view) {
+    const dpr = window.devicePixelRatio || 1;
+    const out: RectangleMarker[] = [];
+    for (const r of view.state.selection.ranges)
+      if (!r.empty) out.push(...snapMarkers(RectangleMarker.forRange(view, CLASS, r), CLASS, dpr));
+    return out;
+  },
+  update: (u) => u.docChanged || u.selectionSet || u.viewportChanged,
+});
+
+/** Use with `drawSelection()`: replaces its selection layer, which is hidden. */
+export function seamlessSelection(): Extension {
+  return [
+    snappedSelectionLayer,
+    EditorView.theme({
+      ".cm-selectionLayer:not(.cm-openidea-selectionLayer)": {
+        display: "none",
+      },
+    }),
+  ];
+}

src/editor/core/cm/theme.ts

@@ -177,7 +177,17 @@ export const openideaTheme = EditorView.theme({
   "&.cm-focused > .cm-scroller > .cm-selectionLayer .cm-selectionBackground": {
     background: "var(--editor-selection)",
   },
-  ".cm-activeLine": { backgroundColor: "var(--editor-caret-row)" },
+  // Row backgrounds go on a pseudo-element under the selection layer: on the line itself they would hide it.
+  ".cm-activeLine": { backgroundColor: "transparent", "--openidea-line-bg": "var(--editor-caret-row)" },
+  ".cm-activeLine, .cm-openidea-lineBg": { position: "relative" },
+  ".cm-activeLine::before, .cm-openidea-lineBg::before": {
+    content: '""',
+    position: "absolute",
+    inset: "0",
+    zIndex: "-1000",
+    background: "var(--openidea-line-bg)",
+    pointerEvents: "none",
+  },
   ".cm-selectionMatch": {
     backgroundColor: "var(--bg-identifier-under-caret, color-mix(in srgb, var(--fg) 9%, transparent))",
   },
@@ -263,7 +273,13 @@ export const openideaTheme = EditorView.theme({
   ".cm-foldPlaceholder:hover": { backgroundColor: "color-mix(in srgb, var(--fg-muted) 24%, transparent)" },
 
   // Indent guides (cm/indentGuides.ts)
-  ".cm-openidea-guides": {
+  // Above the text so a selection, or a mark's background, never covers part of a guide.
+  ".cm-openidea-guides": { position: "relative" },
+  ".cm-openidea-guides::after": {
+    content: '""',
+    position: "absolute",
+    inset: "0",
+    pointerEvents: "none",
     backgroundImage:
       "repeating-linear-gradient(to right, var(--editor-indent-guide) 0 1px, transparent 1px calc(var(--openidea-guide-step) * 1ch))",
     backgroundSize: "calc(var(--openidea-guide-levels) * var(--openidea-guide-step) * 1ch) 100%",