Sanity check gaps on the outside of the workspace

Avoid gaps that are too large around the workspace that you end up
without any usable space for children.

Fixes #4308

sway/tree/workspace.c

@@ -706,6 +706,25 @@ void workspace_add_gaps(struct sway_workspace *ws) {
 	ws->current_gaps.bottom = fmax(0, ws->current_gaps.bottom + ws->gaps_inner);
 	ws->current_gaps.left = fmax(0, ws->current_gaps.left + ws->gaps_inner);
 
+	// 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 = fmax(0, ws->width - MIN_SANE_W);
+		double left_gap_frac = ((double)ws->current_gaps.left /
+			((double)ws->current_gaps.left + (double)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 = fmax(0, ws->height - MIN_SANE_H);
+		double top_gap_frac = ((double) ws->current_gaps.top /
+			((double)ws->current_gaps.top + (double)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;