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diff --git a/popperjs/package/lib/modifiers/flip.js.flow b/popperjs/package/lib/modifiers/flip.js.flow
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+++ b/popperjs/package/lib/modifiers/flip.js.flow
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+// @flow
+import type { Placement, Boundary, RootBoundary } from '../enums';
+import type { ModifierArguments, Modifier, Padding } from '../types';
+import getOppositePlacement from '../utils/getOppositePlacement';
+import getBasePlacement from '../utils/getBasePlacement';
+import getOppositeVariationPlacement from '../utils/getOppositeVariationPlacement';
+import detectOverflow from '../utils/detectOverflow';
+import computeAutoPlacement from '../utils/computeAutoPlacement';
+import { bottom, top, start, right, left, auto } from '../enums';
+import getVariation from '../utils/getVariation';
+
+// eslint-disable-next-line import/no-unused-modules
+export type Options = {
+  mainAxis: boolean,
+  altAxis: boolean,
+  fallbackPlacements: Array<Placement>,
+  padding: Padding,
+  boundary: Boundary,
+  rootBoundary: RootBoundary,
+  altBoundary: boolean,
+  flipVariations: boolean,
+  allowedAutoPlacements: Array<Placement>,
+};
+
+function getExpandedFallbackPlacements(placement: Placement): Array<Placement> {
+  if (getBasePlacement(placement) === auto) {
+    return [];
+  }
+
+  const oppositePlacement = getOppositePlacement(placement);
+
+  return [
+    getOppositeVariationPlacement(placement),
+    oppositePlacement,
+    getOppositeVariationPlacement(oppositePlacement),
+  ];
+}
+
+function flip({ state, options, name }: ModifierArguments<Options>) {
+  if (state.modifiersData[name]._skip) {
+    return;
+  }
+
+  const {
+    mainAxis: checkMainAxis = true,
+    altAxis: checkAltAxis = true,
+    fallbackPlacements: specifiedFallbackPlacements,
+    padding,
+    boundary,
+    rootBoundary,
+    altBoundary,
+    flipVariations = true,
+    allowedAutoPlacements,
+  } = options;
+
+  const preferredPlacement = state.options.placement;
+  const basePlacement = getBasePlacement(preferredPlacement);
+  const isBasePlacement = basePlacement === preferredPlacement;
+
+  const fallbackPlacements =
+    specifiedFallbackPlacements ||
+    (isBasePlacement || !flipVariations
+      ? [getOppositePlacement(preferredPlacement)]
+      : getExpandedFallbackPlacements(preferredPlacement));
+
+  const placements = [preferredPlacement, ...fallbackPlacements].reduce(
+    (acc, placement) => {
+      return acc.concat(
+        getBasePlacement(placement) === auto
+          ? computeAutoPlacement(state, {
+              placement,
+              boundary,
+              rootBoundary,
+              padding,
+              flipVariations,
+              allowedAutoPlacements,
+            })
+          : placement
+      );
+    },
+    []
+  );
+
+  const referenceRect = state.rects.reference;
+  const popperRect = state.rects.popper;
+
+  const checksMap = new Map();
+  let makeFallbackChecks = true;
+  let firstFittingPlacement = placements[0];
+
+  for (let i = 0; i < placements.length; i++) {
+    const placement = placements[i];
+    const basePlacement = getBasePlacement(placement);
+    const isStartVariation = getVariation(placement) === start;
+    const isVertical = [top, bottom].indexOf(basePlacement) >= 0;
+    const len = isVertical ? 'width' : 'height';
+
+    const overflow = detectOverflow(state, {
+      placement,
+      boundary,
+      rootBoundary,
+      altBoundary,
+      padding,
+    });
+
+    let mainVariationSide: any = isVertical
+      ? isStartVariation
+        ? right
+        : left
+      : isStartVariation
+      ? bottom
+      : top;
+
+    if (referenceRect[len] > popperRect[len]) {
+      mainVariationSide = getOppositePlacement(mainVariationSide);
+    }
+
+    const altVariationSide: any = getOppositePlacement(mainVariationSide);
+
+    const checks = [];
+
+    if (checkMainAxis) {
+      checks.push(overflow[basePlacement] <= 0);
+    }
+
+    if (checkAltAxis) {
+      checks.push(
+        overflow[mainVariationSide] <= 0,
+        overflow[altVariationSide] <= 0
+      );
+    }
+
+    if (checks.every((check) => check)) {
+      firstFittingPlacement = placement;
+      makeFallbackChecks = false;
+      break;
+    }
+
+    checksMap.set(placement, checks);
+  }
+
+  if (makeFallbackChecks) {
+    // `2` may be desired in some cases – research later
+    const numberOfChecks = flipVariations ? 3 : 1;
+
+    for (let i = numberOfChecks; i > 0; i--) {
+      const fittingPlacement = placements.find((placement) => {
+        const checks = checksMap.get(placement);
+        if (checks) {
+          return checks.slice(0, i).every((check) => check);
+        }
+      });
+
+      if (fittingPlacement) {
+        firstFittingPlacement = fittingPlacement;
+        break;
+      }
+    }
+  }
+
+  if (state.placement !== firstFittingPlacement) {
+    state.modifiersData[name]._skip = true;
+    state.placement = firstFittingPlacement;
+    state.reset = true;
+  }
+}
+
+// eslint-disable-next-line import/no-unused-modules
+export type FlipModifier = Modifier<'flip', Options>;
+export default ({
+  name: 'flip',
+  enabled: true,
+  phase: 'main',
+  fn: flip,
+  requiresIfExists: ['offset'],
+  data: { _skip: false },
+}: FlipModifier);