feat(popover): add pure anchored-positioning math

2026-06-02 15:59:58 +03:00
parent f3c76df2c5
commit ef9cd33e48
2 changed files with 268 additions and 0 deletions
@@ -0,0 +1,149 @@
+import { clampNumber } from '$shared/lib/utils';
+
+/**
+ * Side of the trigger the content prefers to open toward.
+ */
+export type Side = 'top' | 'bottom' | 'left' | 'right';
+
+/**
+ * Cross-axis alignment of the content relative to the trigger.
+ */
+export type Align = 'start' | 'center' | 'end';
+
+/**
+ * Inputs for a single placement computation. All geometry is injected
+ * (no DOM reads) so the function stays pure and unit-testable.
+ */
+type ComputeArgs = {
+    /**
+     * Trigger bounding rect (viewport coordinates).
+     */
+    triggerRect: DOMRect;
+    /**
+     * Measured content size.
+     */
+    contentRect: { width: number; height: number };
+    /**
+     * Viewport size.
+     */
+    viewport: { width: number; height: number };
+    /**
+     * Preferred side.
+     */
+    side: Side;
+    /**
+     * Cross-axis alignment.
+     */
+    align: Align;
+    /**
+     * Gap between trigger and content on the main axis.
+     */
+    sideOffset: number;
+};
+
+/**
+ * Resolved placement: fixed-position coordinates plus the side actually used
+ * (may differ from the requested side after a flip).
+ */
+type ComputeResult = { x: number; y: number; side: Side };
+
+const OPPOSITE: Record<Side, Side> = {
+    top: 'bottom',
+    bottom: 'top',
+    left: 'right',
+    right: 'left',
+};
+
+/**
+ * True for sides whose main axis is vertical (content sits above/below).
+ */
+function isVertical(side: Side): boolean {
+    return side === 'top' || side === 'bottom';
+}
+
+/**
+ * Main-axis coordinate (top for vertical sides, left for horizontal sides).
+ */
+function mainAxisCoord(side: Side, t: DOMRect, c: { width: number; height: number }, offset: number): number {
+    switch (side) {
+        case 'top':
+            return t.top - c.height - offset;
+        case 'bottom':
+            return t.bottom + offset;
+        case 'left':
+            return t.left - c.width - offset;
+        case 'right':
+            return t.right + offset;
+    }
+}
+
+/**
+ * Whether the content fits on the given side within the viewport.
+ */
+function fitsOnSide(
+    side: Side,
+    t: DOMRect,
+    c: { width: number; height: number },
+    v: { width: number; height: number },
+    offset: number,
+): boolean {
+    const coord = mainAxisCoord(side, t, c, offset);
+    switch (side) {
+        case 'top':
+            return coord >= 0;
+        case 'left':
+            return coord >= 0;
+        case 'bottom':
+            return coord + c.height <= v.height;
+        case 'right':
+            return coord + c.width <= v.width;
+    }
+}
+
+/**
+ * Cross-axis coordinate for the requested alignment.
+ */
+function crossAxisCoord(side: Side, align: Align, t: DOMRect, c: { width: number; height: number }): number {
+    if (isVertical(side)) {
+        if (align === 'start') {
+            return t.left;
+        }
+        if (align === 'end') {
+            return t.right - c.width;
+        }
+        return t.left + t.width / 2 - c.width / 2;
+    }
+    if (align === 'start') {
+        return t.top;
+    }
+    if (align === 'end') {
+        return t.bottom - c.height;
+    }
+    return t.top + t.height / 2 - c.height / 2;
+}
+
+/**
+ * Compute an anchored placement with flip (to the opposite side when the
+ * preferred side doesn't fit but the opposite does) and shift (clamp the
+ * cross axis so the content stays within the viewport).
+ */
+export function computePosition(args: ComputeArgs): ComputeResult {
+    const { triggerRect: t, contentRect: c, viewport: v, align, sideOffset } = args;
+    let side = args.side;
+
+    if (!fitsOnSide(side, t, c, v, sideOffset) && fitsOnSide(OPPOSITE[side], t, c, v, sideOffset)) {
+        side = OPPOSITE[side];
+    }
+
+    let x: number;
+    let y: number;
+    if (isVertical(side)) {
+        y = mainAxisCoord(side, t, c, sideOffset);
+        x = clampNumber(crossAxisCoord(side, align, t, c), 0, Math.max(0, v.width - c.width));
+    } else {
+        x = mainAxisCoord(side, t, c, sideOffset);
+        y = clampNumber(crossAxisCoord(side, align, t, c), 0, Math.max(0, v.height - c.height));
+    }
+
+    return { x, y, side };
+}