diff --git a/e2e/themes.spec.ts b/e2e/themes.spec.ts
index ce717dac..30cc7799 100644
--- a/e2e/themes.spec.ts
+++ b/e2e/themes.spec.ts
@@ -111,7 +111,7 @@ for (const palette of palettes) {
       );
     await editor.locator(".cm-content").click();
     await page.keyboard.press("Meta+a");
-    await expect(editor.locator(".cm-selectionBackground").first()).toBeVisible();
+    await expect(editor.locator(".cm-selectionBackground:visible").first()).toBeVisible();
     await page.keyboard.press("ArrowRight");
     await page.keyboard.press("Meta+f");
     await page.getByRole("textbox", { name: "Find", exact: true }).fill("Order");
diff --git a/src/editor/core/cm/prefs.ts b/src/editor/core/cm/prefs.ts
index 81fd80ee..43501431 100644
--- a/src/editor/core/cm/prefs.ts
+++ b/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(" ") }) : [],
   ];
 }
diff --git a/src/editor/core/cm/selectionLayer.test.ts b/src/editor/core/cm/selectionLayer.test.ts
new file mode 100644
index 00000000..3d489439
--- /dev/null
+++ b/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);
+  });
+});
diff --git a/src/editor/core/cm/selectionLayer.ts b/src/editor/core/cm/selectionLayer.ts
new file mode 100644
index 00000000..034c4589
--- /dev/null
+++ b/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",
+      },
+    }),
+  ];
+}
diff --git a/src/editor/core/cm/theme.ts b/src/editor/core/cm/theme.ts
index ab6727b9..23eb911c 100644
--- a/src/editor/core/cm/theme.ts
+++ b/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%",
diff --git a/src/features/debugger/editor/breakpointGutter.ts b/src/features/debugger/editor/breakpointGutter.ts
index 704f442f..a81be943 100644
--- a/src/features/debugger/editor/breakpointGutter.ts
+++ b/src/features/debugger/editor/breakpointGutter.ts
@@ -84,7 +84,7 @@ class BreakpointMarker extends GutterMarker {
   }
 }
 
-const bpLine = Decoration.line({ class: styles.bpLine });
+const bpLine = Decoration.line({ class: `${styles.bpLine} cm-openidea-lineBg` });
 const DISABLED_LOOKS = new Set<BreakpointLook>(["disabled", "muted", "methodDisabled", "fieldDisabled"]);
 
 const setMarkers = StateEffect.define<RangeSet<BreakpointMarker>>();
diff --git a/src/features/debugger/editor/editor.module.css b/src/features/debugger/editor/editor.module.css
index 24653ac7..1ab48dc6 100644
--- a/src/features/debugger/editor/editor.module.css
+++ b/src/features/debugger/editor/editor.module.css
@@ -126,11 +126,11 @@
 
 /* Beat the caret row (`.cm-activeLine`), as IDEA does; the suspended line wins over a breakpoint. */
 :global(.cm-editor) :global(.cm-line).bpLine {
-  background: var(--bg-breakpoint-line);
+  --openidea-line-bg: var(--bg-breakpoint-line);
 }
 
 :global(.cm-editor) :global(.cm-line).execLine {
-  background: var(--bg-debug-line);
+  --openidea-line-bg: var(--bg-debug-line);
 }
 
 .inlineValue {
diff --git a/src/features/debugger/editor/execution.ts b/src/features/debugger/editor/execution.ts
index 749bce22..3751f4c4 100644
--- a/src/features/debugger/editor/execution.ts
+++ b/src/features/debugger/editor/execution.ts
@@ -61,7 +61,7 @@ export function inlineValuesFor(text: string, values: Record<string, string>): s
 function lineDecorations(state: EditorState, exec: Exec | null): DecorationSet {
   if (!exec || exec.line >= state.doc.lines) return Decoration.none;
   const line = state.doc.line(exec.line + 1);
-  return Decoration.set([Decoration.line({ class: styles.execLine }).range(line.from)]);
+  return Decoration.set([Decoration.line({ class: `${styles.execLine} cm-openidea-lineBg` }).range(line.from)]);
 }
 
 export function executionPoint(ctx: EditorDocContext): Extension {
diff --git a/src/features/notebook/CellEditor.tsx b/src/features/notebook/CellEditor.tsx
index 9089b89f..bcf558b8 100644
--- a/src/features/notebook/CellEditor.tsx
+++ b/src/features/notebook/CellEditor.tsx
@@ -13,6 +13,7 @@ import { bracketMatching, indentOnInput, syntaxHighlighting } from "@codemirror/
 import { openideaTheme } from "@/editor/core/cm/theme";
 import { openideaHighlightStyle } from "@/editor/core/cm/highlight";
 import { selectionMarks } from "@/editor/core/cm/selectionMarks";
+import { seamlessSelection } from "@/editor/core/cm/selectionLayer";
 import { findLanguageDescription, loadSupport } from "@/editor/core/cm/languages";
 import { tx } from "@/platform/i18n";
 import type { NotebookSession } from "./session";
@@ -45,6 +46,7 @@ export function CellEditor({
       lineNumbers(),
       drawSelection(),
       selectionMarks,
+      seamlessSelection(),
       highlightActiveLine(),
       highlightSpecialChars(),
       bracketMatching(),