diff --git a/src/vs/editor/browser/viewParts/lines/viewLine.ts b/src/vs/editor/browser/viewParts/lines/viewLine.ts
index 11a33fd32a3fecc4fbe4bcd81e308bcbc19fffbb..9a7011dc1d40cc06eb4e964fe22e01f8e7f8adf1 100644
--- a/src/vs/editor/browser/viewParts/lines/viewLine.ts
+++ b/src/vs/editor/browser/viewParts/lines/viewLine.ts
@@ -42,6 +42,8 @@ const canUseFastRenderedViewLine = (function () {
 	return true;
 })();
 
+let monospaceAssumptionsAreValid = true;
+
 const alwaysRenderInlineSelection = (browser.isEdge);
 
 export class DomReadingContext {
@@ -248,7 +250,7 @@ export class ViewLine implements IVisibleLine {
 		sb.appendASCIIString('</div>');
 
 		let renderedViewLine: IRenderedViewLine | null = null;
-		if (canUseFastRenderedViewLine && lineData.isBasicASCII && options.useMonospaceOptimizations && output.containsForeignElements === ForeignElementType.None) {
+		if (monospaceAssumptionsAreValid && canUseFastRenderedViewLine && lineData.isBasicASCII && options.useMonospaceOptimizations && output.containsForeignElements === ForeignElementType.None) {
 			if (lineData.content.length < 300 && renderLineInput.lineTokens.getCount() < 100) {
 				// Browser rounding errors have been observed in Chrome and IE, so using the fast
 				// view line only for short lines. Please test before removing the length check...
@@ -304,6 +306,29 @@ export class ViewLine implements IVisibleLine {
 		return this._renderedViewLine.getWidthIsFast();
 	}
 
+	public needsMonospaceFontCheck(): boolean {
+		if (!this._renderedViewLine) {
+			return false;
+		}
+		return (this._renderedViewLine instanceof FastRenderedViewLine);
+	}
+
+	public monospaceAssumptionsAreValid(): boolean {
+		if (!this._renderedViewLine) {
+			return monospaceAssumptionsAreValid;
+		}
+		if (this._renderedViewLine instanceof FastRenderedViewLine) {
+			return this._renderedViewLine.monospaceAssumptionsAreValid();
+		}
+		return monospaceAssumptionsAreValid;
+	}
+
+	public onMonospaceAssumptionsInvalidated(): void {
+		if (this._renderedViewLine && this._renderedViewLine instanceof FastRenderedViewLine) {
+			this._renderedViewLine = this._renderedViewLine.toSlowRenderedLine();
+		}
+	}
+
 	public getVisibleRangesForRange(startColumn: number, endColumn: number, context: DomReadingContext): VisibleRanges | null {
 		if (!this._renderedViewLine) {
 			return null;
@@ -382,6 +407,24 @@ class FastRenderedViewLine implements IRenderedViewLine {
 		return true;
 	}
 
+	public monospaceAssumptionsAreValid(): boolean {
+		if (!this.domNode) {
+			return monospaceAssumptionsAreValid;
+		}
+		const expectedWidth = this.getWidth();
+		const actualWidth = (<HTMLSpanElement>this.domNode.domNode.firstChild).offsetWidth;
+		if (Math.abs(expectedWidth - actualWidth) >= 2) {
+			// more than 2px off
+			console.warn(`monospace assumptions have been violated, therefore disabling monospace optimizations!`);
+			monospaceAssumptionsAreValid = false;
+		}
+		return monospaceAssumptionsAreValid;
+	}
+
+	public toSlowRenderedLine(): RenderedViewLine {
+		return createRenderedLine(this.domNode, this.input, this._characterMapping, false, ForeignElementType.None);
+	}
+
 	public getVisibleRangesForRange(startColumn: number, endColumn: number, context: DomReadingContext): HorizontalRange[] | null {
 		const startPosition = this._getCharPosition(startColumn);
 		const endPosition = this._getCharPosition(endColumn);
diff --git a/src/vs/editor/browser/viewParts/lines/viewLines.ts b/src/vs/editor/browser/viewParts/lines/viewLines.ts
index b938a5fd9fa882be8588f7df5272d27091cf227a..09ee4fb551603c6271950a1ae44203bf60cc6a22 100644
--- a/src/vs/editor/browser/viewParts/lines/viewLines.ts
+++ b/src/vs/editor/browser/viewParts/lines/viewLines.ts
@@ -4,6 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 
 import 'vs/css!./viewLines';
+import * as platform from 'vs/base/common/platform';
 import { FastDomNode } from 'vs/base/browser/fastDomNode';
 import { RunOnceScheduler } from 'vs/base/common/async';
 import { Configuration } from 'vs/editor/browser/config/configuration';
@@ -106,6 +107,7 @@ export class ViewLines extends ViewPart implements IVisibleLinesHost<ViewLine>,
 	// --- width
 	private _maxLineWidth: number;
 	private readonly _asyncUpdateLineWidths: RunOnceScheduler;
+	private readonly _asyncCheckMonospaceFontAssumptions: RunOnceScheduler;
 
 	private _horizontalRevealRequest: HorizontalRevealRequest | null;
 	private readonly _lastRenderedData: LastRenderedData;
@@ -140,6 +142,9 @@ export class ViewLines extends ViewPart implements IVisibleLinesHost<ViewLine>,
 		this._asyncUpdateLineWidths = new RunOnceScheduler(() => {
 			this._updateLineWidthsSlow();
 		}, 200);
+		this._asyncCheckMonospaceFontAssumptions = new RunOnceScheduler(() => {
+			this._checkMonospaceFontAssumptions();
+		}, 2000);
 
 		this._lastRenderedData = new LastRenderedData();
 
@@ -148,6 +153,7 @@ export class ViewLines extends ViewPart implements IVisibleLinesHost<ViewLine>,
 
 	public dispose(): void {
 		this._asyncUpdateLineWidths.dispose();
+		this._asyncCheckMonospaceFontAssumptions.dispose();
 		super.dispose();
 	}
 
@@ -513,6 +519,37 @@ export class ViewLines extends ViewPart implements IVisibleLinesHost<ViewLine>,
 		return allWidthsComputed;
 	}
 
+	private _checkMonospaceFontAssumptions(): void {
+		// Problems with monospace assumptions are more apparent for longer lines,
+		// as small rounding errors start to sum up, so we will select the longest
+		// line for a closer inspection
+		let longestLineNumber = -1;
+		let longestWidth = -1;
+		const rendStartLineNumber = this._visibleLines.getStartLineNumber();
+		const rendEndLineNumber = this._visibleLines.getEndLineNumber();
+		for (let lineNumber = rendStartLineNumber; lineNumber <= rendEndLineNumber; lineNumber++) {
+			const visibleLine = this._visibleLines.getVisibleLine(lineNumber);
+			if (visibleLine.needsMonospaceFontCheck()) {
+				const lineWidth = visibleLine.getWidth();
+				if (lineWidth > longestWidth) {
+					longestWidth = lineWidth;
+					longestLineNumber = lineNumber;
+				}
+			}
+		}
+
+		if (longestLineNumber === -1) {
+			return;
+		}
+
+		if (!this._visibleLines.getVisibleLine(longestLineNumber).monospaceAssumptionsAreValid()) {
+			for (let lineNumber = rendStartLineNumber; lineNumber <= rendEndLineNumber; lineNumber++) {
+				const visibleLine = this._visibleLines.getVisibleLine(lineNumber);
+				visibleLine.onMonospaceAssumptionsInvalidated();
+			}
+		}
+	}
+
 	public prepareRender(): void {
 		throw new Error('Not supported');
 	}
@@ -571,6 +608,18 @@ export class ViewLines extends ViewPart implements IVisibleLinesHost<ViewLine>,
 			this._asyncUpdateLineWidths.schedule();
 		}
 
+		if (platform.isLinux && !this._asyncCheckMonospaceFontAssumptions.isScheduled()) {
+			const rendStartLineNumber = this._visibleLines.getStartLineNumber();
+			const rendEndLineNumber = this._visibleLines.getEndLineNumber();
+			for (let lineNumber = rendStartLineNumber; lineNumber <= rendEndLineNumber; lineNumber++) {
+				const visibleLine = this._visibleLines.getVisibleLine(lineNumber);
+				if (visibleLine.needsMonospaceFontCheck()) {
+					this._asyncCheckMonospaceFontAssumptions.schedule();
+					break;
+				}
+			}
+		}
+
 		// (3) handle scrolling
 		this._linesContent.setLayerHinting(this._canUseLayerHinting);
 		this._linesContent.setContain('strict');