Limit outer gaps to sensible values

Just like with inner gaps, outer gaps must be capped so that the
workspace doesn't run out of space. Since here we have different gaps in
all four directions clamp them in a way that at least maintains the
relative sizing of the gaps vertically and horizontally.

Fixes #4300

sway/tree/workspace.c

@@ -748,19 +748,36 @@ void workspace_add_gaps(struct sway_workspace *ws) {
 		// to match the half gaps that the children have all already added around
 		// themselves. We use the opposite order for the two halves so that the
 		// sum adds up to the correct total gap size in all circumstances.
-		int gaps_horizontal = MAX(0, MIN(ws->gaps_inner, ws->width - MIN_SANE_W));
-		int gaps_horizontal1 = gaps_horizontal / 2;
-		int gaps_horizontal2 = gaps_horizontal - gaps_horizontal1;
+		int gaps_horizontal1 = ws->gaps_inner / 2;
+		int gaps_horizontal2 = ws->gaps_inner - gaps_horizontal1;
 		ws->current_gaps.left += gaps_horizontal2;
 		ws->current_gaps.right += gaps_horizontal1;
 
-		int gaps_vertical = MAX(0, MIN(ws->gaps_inner, ws->height - MIN_SANE_H));
-		int gaps_vertical1 = gaps_vertical / 2;
-		int gaps_vertical2 = gaps_vertical - gaps_vertical1;
+		int gaps_vertical1 = ws->gaps_inner / 2;
+		int gaps_vertical2 = ws->gaps_inner - gaps_vertical1;
 		ws->current_gaps.top += gaps_vertical2;
 		ws->current_gaps.bottom += gaps_vertical1;
 	}
 
+	// Now that we have the total gaps calculated we may need to clamp them in
+	// case they've made the available area too small
+	if (ws->width - ws->current_gaps.left - ws->current_gaps.right < MIN_SANE_W &&
+			ws->current_gaps.left + ws->current_gaps.right > 0) {
+		int total_gap = MAX(0, ws->width - MIN_SANE_W);
+		double left_gap_frac = ((float) ws->current_gaps.left /
+			((float) ws->current_gaps.left + (float) ws->current_gaps.right));
+		ws->current_gaps.left = left_gap_frac * total_gap;
+		ws->current_gaps.right = total_gap - ws->current_gaps.left;
+	}
+	if (ws->height - ws->current_gaps.top - ws->current_gaps.bottom < MIN_SANE_H &&
+			ws->current_gaps.top + ws->current_gaps.bottom > 0) {
+		int total_gap = MAX(0, ws->height - MIN_SANE_H);
+		double top_gap_frac = ((float) ws->current_gaps.top /
+			((float) ws->current_gaps.top + (float) ws->current_gaps.bottom));
+		ws->current_gaps.top = top_gap_frac * total_gap;
+		ws->current_gaps.bottom = total_gap - ws->current_gaps.top;
+	}
+
 	ws->x += ws->current_gaps.left;
 	ws->y += ws->current_gaps.top;
 	ws->width -= ws->current_gaps.left + ws->current_gaps.right;