src/vs/editor/browser/viewParts/selections/selections.ts/SelectionsOverlay/_enrichVisibleRangesWithStyle
class SelectionsOverlay:	private _enrichVisibleRangesWithStyle(viewport: Range, linesVisibleRanges: LineVisibleRangesWithStyle[], previousFrame: LineVisibleRangesWithStyle[] | null): void {
		const epsilon = this._typicalHalfwidthCharacterWidth / 4;
		let previousFrameTop: HorizontalRangeWithStyle | null = null;
		let previousFrameBottom: HorizontalRangeWithStyle | null = null;

		if (previousFrame && previousFrame.length > 0 && linesVisibleRanges.length > 0) {

			const topLineNumber = linesVisibleRanges[0].lineNumber;
			if (topLineNumber === viewport.startLineNumber) {
				for (let i = 0; !previousFrameTop && i < previousFrame.length; i++) {
					if (previousFrame[i].lineNumber === topLineNumber) {
						previousFrameTop = previousFrame[i].ranges[0];
					}
				}
			}

			const bottomLineNumber = linesVisibleRanges[linesVisibleRanges.length - 1].lineNumber;
			if (bottomLineNumber === viewport.endLineNumber) {
				for (let i = previousFrame.length - 1; !previousFrameBottom && i >= 0; i--) {
					if (previousFrame[i].lineNumber === bottomLineNumber) {
						previousFrameBottom = previousFrame[i].ranges[0];
					}
				}
			}

			if (previousFrameTop && !previousFrameTop.startStyle) {
				previousFrameTop = null;
			}
			if (previousFrameBottom && !previousFrameBottom.startStyle) {
				previousFrameBottom = null;
			}
		}

		for (let i = 0, len = linesVisibleRanges.length; i < len; i++) {
			// We know for a fact that there is precisely one range on each line
			const curLineRange = linesVisibleRanges[i].ranges[0];
			const curLeft = curLineRange.left;
			const curRight = curLineRange.left + curLineRange.width;

			const startStyle = {
				top: CornerStyle.EXTERN,
				bottom: CornerStyle.EXTERN
			};

			const endStyle = {
				top: CornerStyle.EXTERN,
				bottom: CornerStyle.EXTERN
			};

			if (i > 0) {
				// Look above
				const prevLeft = linesVisibleRanges[i - 1].ranges[0].left;
				const prevRight = linesVisibleRanges[i - 1].ranges[0].left + linesVisibleRanges[i - 1].ranges[0].width;

				if (abs(curLeft - prevLeft) < epsilon) {
					startStyle.top = CornerStyle.FLAT;
				} else if (curLeft > prevLeft) {
					startStyle.top = CornerStyle.INTERN;
				}

				if (abs(curRight - prevRight) < epsilon) {
					endStyle.top = CornerStyle.FLAT;
				} else if (prevLeft < curRight && curRight < prevRight) {
					endStyle.top = CornerStyle.INTERN;
				}
			} else if (previousFrameTop) {
				// Accept some hiccups near the viewport edges to save on repaints
				startStyle.top = previousFrameTop.startStyle!.top;
				endStyle.top = previousFrameTop.endStyle!.top;
			}

			if (i + 1 < len) {
				// Look below
				const nextLeft = linesVisibleRanges[i + 1].ranges[0].left;
				const nextRight = linesVisibleRanges[i + 1].ranges[0].left + linesVisibleRanges[i + 1].ranges[0].width;

				if (abs(curLeft - nextLeft) < epsilon) {
					startStyle.bottom = CornerStyle.FLAT;
				} else if (nextLeft < curLeft && curLeft < nextRight) {
					startStyle.bottom = CornerStyle.INTERN;
				}

				if (abs(curRight - nextRight) < epsilon) {
					endStyle.bottom = CornerStyle.FLAT;
				} else if (curRight < nextRight) {
					endStyle.bottom = CornerStyle.INTERN;
				}
			} else if (previousFrameBottom) {
				// Accept some hiccups near the viewport edges to save on repaints
				startStyle.bottom = previousFrameBottom.startStyle!.bottom;
				endStyle.bottom = previousFrameBottom.endStyle!.bottom;
			}

			curLineRange.startStyle = startStyle;
			curLineRange.endStyle = endStyle;
		}
	}